Light-emitting element

ABSTRACT

A light-emitting element having high external quantum efficiency is provided. A light-emitting element having a long lifetime is provided. A light-emitting element is provided which includes a light-emitting layer containing a phosphorescent compound, a first organic compound, and a second organic compound between a pair of electrodes, in which a combination of the first organic compound and the second organic compound forms an exciplex (excited complex). The light-emitting element transfers energy by utilizing an overlap between the emission spectrum of the exciplex and the absorption spectrum of the phosphorescent compound and thus has high energy transfer efficiency. Therefore, a light-emitting element having high external quantum efficiency can be obtained.

TECHNICAL FIELD

The present invention relates to light-emitting elements using anorganic electroluminescence (EL) phenomenon (hereinafter suchlight-emitting elements are also referred to as organic EL elements).

BACKGROUND ART

An organic EL element has been actively researched and developed. In afundamental structure of the organic EL element, a layer including aluminescent organic compound (hereinafter also referred to aslight-emitting layer) is interposed between a pair of electrodes. Theorganic EL element has attracted attention as a next-generation flatpanel display element owing to characteristics such as feasibility ofbeing thinner and lighter, high speed response to input signals, andcapability of direct current low voltage driving. In addition, a displayusing such a light-emitting element has a feature that it is excellentin contrast and image quality, and has a wide viewing angle. Further,being a planar light source, the organic EL element has been attemptedto be applied as a light source such as a backlight of a liquid crystaldisplay and a lighting device.

The emission mechanism of the organic EL element is of acarrier-injection type. That is, by application of voltage with alight-emitting layer interposed between electrodes, electrons and holesinjected from the electrodes are recombined to make a light-emittingsubstance excited, and light is emitted when the excited state relaxesto the ground state. There can be two types of the excited states: asinglet excited state (S*) and a triplet excited state (T*). Thestatistical generation ratio of the excited states in a light-emittingelement is considered to be S*:T*=1:3.

In general, the ground state of a light-emitting organic compound is asinglet state. Therefore, light emission from the singlet excited state(S*) is referred to as fluorescence because it is caused by electrontransition between the same spin multiplicities. On the other hand,light emission from the triplet excited state (T*) is referred to asphosphorescence where electron transition occurs between different spinmultiplicities. Here, in a compound emitting fluorescence (hereinafterreferred to as fluorescent compound), in general, phosphorescence is notobserved at room temperature, and only fluorescence is observed.Accordingly, the internal quantum efficiency (the ratio of generatedphotons to injected carriers) in a light-emitting element including afluorescent compound is assumed to have a theoretical limit of 25% basedon S*:T*=1:3.

On the other hand, when a compound emitting phosphorescence (hereinafterreferred to as phosphorescent compound) is used, an internal quantumefficiency of 100% can be theoretically achieved. That is, higheremission efficiency can be obtained than using a fluorescent compound.For these reasons, a light-emitting element including a phosphorescentcompound has been actively developed in recent years in order to achievea high-efficiency light-emitting element. As the phosphorescentcompound, an organometallic complex that has iridium or the like as acentral metal has particularly attracted attention owing to their highphosphorescence quantum yield; for example, an organometallic complexthat has iridium as a central metal is disclosed as a phosphorescentmaterial in Patent Document 1.

When a light-emitting layer of a light-emitting element is formed usinga phosphorescent compound described above, in order to suppressconcentration quenching or quenching due to triplet-triplet annihilationin the phosphorescent compound, the light-emitting layer is often formedsuch that the phosphorescent compound is dispersed in a matrix ofanother compound. Here, the compound serving as the matrix is calledhost material, and the compound dispersed in the matrix, such as aphosphorescent compound, is called guest material.

REFERENCE Patent Document

-   [Patent Document 1] PCT International Publication No. 00/70655

DISCLOSURE OF INVENTION

However, it is generally said that the light extraction efficiency of anorganic EL element is approximately 20% to 30%. Accordingly, consideringlight absorption by a reflective electrode and a transparent electrode,the external quantum efficiency of a light-emitting element including aphosphorescent compound has a limit of approximately 25% at most.

An object of one embodiment of the present invention is to provide alight-emitting element with high external quantum efficiency. Anotherobject of one embodiment of the present invention is to provide alight-emitting element having a long lifetime.

One embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex.

Another embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex, andthe exciplex acts on the phosphorescent compound so that thephosphorescent compound emits phosphorescence.

Another embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which an exciplex is formed from asinglet exciton of the first organic compound.

Another embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which an exciplex is formed from ananion of the first organic compound and a cation of the second organiccompound.

In the aforementioned light-emitting element, it is preferable thatexcitation energy of the exciplex be transferred to the phosphorescentcompound so that the phosphorescent compound emits phosphorescence.

In the aforementioned light-emitting element, it is preferable that atleast one of the first organic compound and the second organic compoundbe a fluorescent compound.

In the aforementioned light-emitting element, it is preferable that thephosphorescent compound be an organometallic complex.

The light-emitting element of one embodiment of the present inventioncan be applied to a light-emitting device, an electronic device, and alighting device.

According to one embodiment of the present invention, a light-emittingelement having high external quantum efficiency can be provided.According to another embodiment of the present invention, alight-emitting element having a long lifetime can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B each show an absorption spectrum and emission spectraaccording to Example 1.

FIGS. 2A and 2B each show an absorption spectrum and emission spectrumaccording to Example 2.

FIG. 3 shows current density-luminance characteristics of alight-emitting element of Example 3.

FIG. 4 shows voltage-luminance characteristics of the light-emittingelement of Example 3.

FIG. 5 shows luminance-current efficiency characteristics of thelight-emitting element of Example 3.

FIG. 6 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 3.

FIG. 7 shows an emission spectrum of the light-emitting element ofExample 3.

FIG. 8 shows results of reliability tests of the light-emitting elementof Example 3.

FIG. 9 shows current density-luminance characteristics of alight-emitting element of Example 4.

FIG. 10 shows voltage-luminance characteristics of the light-emittingelement of Example 4.

FIG. 11 shows luminance-current efficiency characteristics of thelight-emitting element of Example 4.

FIG. 12 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 4.

FIG. 13 shows an emission spectrum of the light-emitting element ofExample 4.

FIG. 14 shows results of reliability tests of the light-emitting elementof Example 4.

FIG. 15 shows the structure of light-emitting elements of examples.

FIGS. 16A to 16C each illustrate a light-emitting element of oneembodiment of the present invention.

FIG. 17 illustrates energy levels of an exciplex applied to oneembodiment of the present invention.

FIGS. 18A and 18B each show an absorption spectrum and emission spectraaccording to Example 5.

FIG. 19 shows current density-luminance characteristics of alight-emitting element of Example 6.

FIG. 20 shows voltage-luminance characteristics of the light-emittingelement of Example 6.

FIG. 21 shows luminance-current efficiency characteristics of thelight-emitting element of Example 6.

FIG. 22 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 6.

FIG. 23 shows an emission spectrum of the light-emitting element ofExample 6.

FIG. 24 shows results of reliability tests of the light-emitting elementof Example 6.

FIG. 25 illustrates a concept of one embodiment of the presentinvention.

FIGS. 26A and 26B each show an absorption spectrum and emission spectraaccording to Example 7.

FIG. 27 shows current density-luminance characteristics of a lightemitting element of Example 8.

FIG. 28 shows voltage-luminance characteristics of the light-emittingelement of Example 8.

FIG. 29 shows luminance-current efficiency characteristics of thelight-emitting element of Example 8.

FIG. 30 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 8.

FIG. 31 shows an emission spectrum of the light-emitting element ofExample 8.

FIGS. 32A and 32B each show an absorption spectrum and emission spectraaccording to Example 9.

FIG. 33 shows current density-luminance characteristics of alight-emitting element of Example 10.

FIG. 34 shows voltage-luminance characteristics of the light-emittingelement of Example 10.

FIG. 35 shows luminance-current efficiency characteristics of thelight-emitting element of Example 10.

FIG. 36 shows luminance-external quantum efficiency characteristics ofthe light-emitting element of Example 10.

FIG. 37 shows an emission spectrum of the light-emitting element ofExample 10.

FIG. 38 shows results of reliability tests of the light-emitting elementof Example 10.

FIGS. 39A and 39B each show an absorption and emission spectra accordingto Example 11.

FIG. 40 shows current density-luminance characteristics oflight-emitting elements of Example 12.

FIG. 41 shows voltage-luminance characteristics of the light-emittingelements of Example 12.

FIG. 42 shows luminance-current efficiency characteristics of thelight-emitting elements of Example 12.

FIG. 43 shows luminance-external quantum efficiency characteristics ofthe light-emitting elements of Example 12.

FIG. 44 shows emission spectra of the light-emitting elements of Example12.

FIG. 45 shows results of reliability tests of the light-emittingelements of Example 12.

FIGS. 46A and 46B each show an absorption spectrum and emission spectraaccording to Example 13.

FIG. 47 shows current density-luminance characteristics oflight-emitting elements of Example 14.

FIG. 48 shows voltage-luminance characteristics of the light-emittingelements of Example 14.

FIG. 49 illustrates luminance-current efficiency characteristics of thelight-emitting elements of Example 14.

FIG. 50 shows luminance-external quantum efficiency characteristics ofthe light-emitting elements of Example 14.

FIG. 51 shows emission spectra of the light-emitting elements of Example14.

FIGS. 52A and 52B each show an absorption spectrum and emission spectraaccording to Example 15.

FIG. 53 shows current density-luminance characteristics oflight-emitting elements of Example 16.

FIG. 54 shows voltage-luminance characteristics of the light-emittingelements of Example 16.

FIG. 55 shows luminance-current efficiency characteristics of thelight-emitting elements of Example 16.

FIG. 56 shows luminance-external quantum efficiency characteristics ofthe light-emitting elements of Example 16.

FIG. 57 shows emission spectra of the light-emitting elements of Example16.

FIGS. 58A and 58B each show an absorption spectrum and emission spectrumaccording to Example 17.

FIG. 59 shows current density-luminance characteristics oflight-emitting elements of Example 18.

FIG. 60 shows voltage-luminance characteristics of the light-emittingelements of Example 18.

FIG. 61 shows luminance-current efficiency characteristics of thelight-emitting elements of Example 18.

FIG. 62 shows luminance-external quantum efficiency characteristics ofthe light-emitting elements of Example 18.

FIG. 63 shows emission spectra of the light-emitting elements of Example18.

FIG. 64 shows calculation results according to one embodiment of thepresent invention.

FIGS. 65(A1), 65(A2), 65(B1), 65(B2), 65(C1), and 65(C2) showcalculation results according to one embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described with reference to drawings. Note that theinvention is not limited to the following description, and it will beeasily understood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe invention. Therefore, the invention should not be construed as beinglimited to the description in the following embodiments. Note that inthe structures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and description of such portions is notrepeated.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described.

The light-emitting element of this embodiment includes a light-emittinglayer containing a guest material as a light-emitting substance, a firstorganic compound, and a second organic compound. Specifically, aphosphorescent compound is used as the guest material. Note that one ofthe first and second organic compounds, the content of which is higherthan that of the other in the light-emitting layer, is called hostmaterial.

The structure in which the guest material is dispersed in the hostmaterial can prevent the light-emitting layer from crystallizing.Further, it is possible to suppress concentration quenching due to highconcentration of the guest material, and thus the light-emitting elementcan have higher emission efficiency.

Note that in this embodiment, it is preferable that the level of atriplet excitation energy (T₁ level) of each of the first and secondorganic compounds be higher than that of the guest material. This isbecause, when the T₁ level of the first organic compound (or the secondorganic compound) is lower than that of the guest material, the tripletexcitation energy of the guest material, which is to contribute to lightemission, is quenched by the first organic compound (or the secondorganic compound) and accordingly the emission efficiency is decreased.

<Elementary Processes of Light Emission>

First, a description is given of general elementary processes of lightemission in a light-emitting element using a phosphorescent compound asa guest material.

(1) The case where an electron and a hole are recombined in a guestmolecule, and the guest molecule is excited (direct recombinationprocess).

(1-1) When the excited state of the guest molecule is a triplet excitedstate, the guest molecule emits phosphorescence.

(1-2) When the excited state of the guest molecule is a singlet excitedstate, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state and emitsphosphorescence.

In other words, in the direct recombination process in (1), as long asthe efficiency of intersystem crossing and the phosphorescence quantumyield of the guest molecule are high, high emission efficiency can beobtained. Note that as described above, the T₁ level of the hostmolecule is preferably higher than the T₁ level of the guest molecule.

(2) The case where an electron and a hole are recombined in a hostmolecule and the host molecule is put in an excited state (energytransfer process).

(2-1) When the excited state of the host molecule is a triplet excitedstate and the T₁ level of the host molecule is higher than that of theguest molecule, excitation energy is transferred from the host moleculeto the guest molecule, and thus the guest molecule is put in a tripletexcited state. The guest molecule in the triplet excited state emitsphosphorescence. Note that energy transfer to a singlet excitationenergy level (S₁ level) of the guest molecule can occur in theory, butis unlikely to be a main energy transfer process because, in many cases,the S₁ level of the guest molecule has a higher energy than the T₁ levelof the host molecule; therefore, a description thereof is not givenhere.

(2-2) When the excited state of the host molecule is a singlet excitedstate and the S₁ level of the host molecule is higher than the S₁ leveland T₁ level of the guest molecule, excitation energy is transferredfrom the host molecule to the guest molecule, and thus, the guestmolecule is put in a singlet excited state or a triplet excited state.The guest molecule in the triplet excited state emits phosphorescence.In addition, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state, and emitsphosphorescence.

In other words, in the energy transfer process in (2), it is importanthow efficiently both the triplet excitation energy and the singletexcitation energy of the host molecule can transfer to the guestmolecule.

In view of the above-described energy transfer processes, before theexcitation energy of the host molecule is transferred to the guestmolecule, when the host molecule itself is deactivated by emitting theexcitation energy as light or heat, the emission efficiency isdecreased. The inventors have found out that when the host molecule isin a singlet excited state (the above (2-2)), the energy is unlikely totransfer to the guest molecule, i.e., the phosphorescent compound, andthe emission efficiency is likely to be decreased as compared to whenthe host molecule is in a triplet excited state (the above (2-1)). Thus,the inventors have focused on that fact as an object. The reason hasbeen found as follows in consideration of a more detailed energytransfer process.

<Energy Transfer Process>

The following describes energy transfer processes between molecules indetail.

First, as a mechanism of energy transfer between molecules, thefollowing two mechanisms are proposed. A molecule providing excitationenergy is referred to as host molecule, while a molecule receiving theexcitation energy is referred to as guest molecule.

<<Förster Mechanism (Dipole-Dipole Interaction)>>

In Förster mechanism (also referred to as Förster resonance energytransfer), direct intermolecular contact is not necessary for energytransfer. Through a resonant phenomenon of dipolar oscillation between ahost molecule and a guest molecule, energy transfer occurs. By theresonant phenomenon of dipolar oscillation, the host molecule providesenergy to the guest molecule, and thus, the host molecule is put in aground state and the guest molecule is put in an excited state. The rateconstant k_(h*→g) of Förster mechanism is expressed by Formula (1).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} (1)} \right\rbrack \mspace{605mu}} & \; \\{k_{h^{*}\rightarrow g} = {\frac{9000\mspace{11mu} c^{4}K^{2}{\varphi ln10}}{128\mspace{11mu} \pi^{5}n^{\; 4}{N\tau R}^{\; 6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{\; 4}}\ {dv}}}}} & (1)\end{matrix}$

In Formula (1), ν denotes a frequency, f_(h)(ν) denotes a normalizedemission spectrum of a host molecule (a fluorescent spectrum in energytransfer from a singlet excited state, and a phosphorescent spectrum inenergy transfer from a triplet excited state), ε_(g)(ν) denotes a molarabsorption coefficient of a guest molecule, N denotes Avogadro's number,n denotes a refractive index of a medium, R denotes an intermoleculardistance between the host molecule and the guest molecule, r denotes ameasured lifetime of an excited state (fluorescence lifetime orphosphorescence lifetime), c denotes the speed of light, ϕ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host molecule and the guest molecule. Note that K²=⅔ inrandom orientation.

<<Dexter Mechanism (Electron Exchange Interaction)>>

In Dexter mechanism (also referred to as Dexter electron transfer), ahost molecule and a guest molecule are close to a contact effectiverange where their orbitals overlap, and the host molecule in an excitedstate and the guest molecule in a ground state exchange their electrons,which leads to energy transfer. The rate constant k_(h*→g) of Dextermechanism is expressed by Formula (2).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} (2)} \right\rbrack \mspace{605mu}} & \; \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp\left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}\ {dv}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescent spectrumin energy transfer from a singlet excited state, and a phosphorescentspectrum in energy transfer from a triplet excited state), ε′_(g)(ν)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is thought to beexpressed by Formula (3). In the formula, k_(r) denotes a rate constantof a light-emission process (fluorescence in energy transfer from asinglet excited state, and phosphorescence in energy transfer from atriplet excited state) of a host molecule, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of a host molecule, and τ denotes a measured lifetime of anexcited state of a host molecule.

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} (3)} \right\rbrack \mspace{605mu}} & \; \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

First, according to Formula (3), it is found that the energy transferefficiency Φ_(ET) can be increased by further increasing the rateconstant k_(h*→g) of energy transfer as compared with another competingrate constant kr+kn (=1/τ). Then, in order to increase the rate constantk_(h*→g) of energy transfer, based on Formulae (1) and (2), in Förstermechanism and Dexter mechanism, it is preferable that an emissionspectrum of a host molecule (a fluorescent spectrum in energy transferfrom a singlet excited state, and a phosphorescent spectrum in energytransfer from a triplet excited state) largely overlap with anabsorption spectrum of a guest molecule.

Here, the present inventors have considered that the absorption band onthe longest wavelength side (lowest energy side) in the absorptionspectrum of the guest molecule is important in considering the overlapbetween the emission spectrum of the host molecule and the absorptionspectrum of the guest molecule.

In this embodiment, a phosphorescent compound is used as the guestmaterial. In an absorption spectrum of the phosphorescent compound, anabsorption band that is considered to contribute to light emission mostgreatly is an absorption wavelength corresponding to direct transitionfrom a singlet ground state to a triplet excitation state and a vicinityof the absorption wavelength, which is on the longest wavelength side.Therefore, it is considered preferable that the emission spectrum (afluorescent spectrum and a phosphorescent spectrum) of the host materialoverlap with the absorption band on the longest wavelength side in theabsorption spectrum of the phosphorescent compound.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band at around 500 nm to 600nm as the absorption band on the longest wavelength side (as a matter offact, the broad absorption band can be on a shorter or longer wavelengthside depending on emission wavelengths). This absorption band is mainlybased on a triplet MLCT (metal to ligand charge transfer) transition.Note that it is considered that the absorption band also includesabsorptions based on a triplet π-π* transition and a singlet MLCTtransition, and that these absorptions overlap each another to form abroad absorption band on the longest wavelength side in the absorptionspectrum. In other words, the difference between the lowest singletexcited state and the lowest triplet excited state is small, andabsorptions based on these states overlap each other to form a broadabsorption band on the longest wavelength side in the absorptionspectrum. Therefore, as described above, it is preferable that the broadabsorption band on the longest wavelength side largely overlap with theemission spectrum of the host material when an organometallic complex(especially iridium complex) is used as the guest material.

Here, first, energy transfer from a host material in a triplet excitedstate will be considered. From the above-described discussion, it ispreferable that, in energy transfer from a triplet excited state, thephosphorescent spectrum of the host material and the absorption band onthe longest wavelength side of the guest material largely overlap eachother.

Note that a fluorescent compound is generally used as the host material;thus, phosphorescence lifetime (τ) is a millisecond or longer which isextremely long (i.e., k_(r)+k_(n) is low). This is because thetransition from the triplet excited state to the ground state (singlet)is a forbidden transition. Formula (3) shows that this is favorable toenergy transfer efficiency Φ_(ET). This also suggests that energy isgenerally likely to be transferred from the host material in the tripletexcited state to the guest material in the triplet excited state.

However, a question here is energy transfer from the host material inthe singlet excited state. In order to efficiently perform not onlyenergy transfer from the triplet excited state but also energy transferfrom the singlet excited state, it is clear from the above-describeddiscussion that the host material needs to be designed so as to have notonly its phosphorescent spectrum but also its fluorescent spectrumoverlapping with the absorption band on the longest wavelength side ofthe guest material. In other words, unless the host material is designedso as to have its fluorescent spectrum in a position similar to that ofits phosphorescent spectrum, it is not possible to achieve efficientenergy transfer from the host material in both the singlet excited stateand the triplet excited state.

However, the S₁ level generally differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used as a host material in a light-emitting element including aphosphorescent compound, has a phosphorescent spectrum at around 500 nmand has a fluorescent spectrum at around 400 nm, which are largelydifferent by about 100 nm. This example also shows that it is extremelydifficult to design a host material so as to have its fluorescentspectrum in a position similar to that of its phosphorescent spectrum.Therefore, the present inventors consider it a major challenge toimprove efficiency in energy transfer from the host material in thesinglet excited state to the guest material.

Note that fluorescence lifetime (τ) of a fluorescent compound that isused as the host material is on the order of nanoseconds which isextremely short (i.e., k_(r)+k_(n) is high). This is because thetransition from the singlet excited state to the ground state (singlet)is an allowed transition. Formula (3) shows that this is unfavorable toenergy transfer efficiency Φ_(ET). This also suggests that energy isgenerally unlikely to be transferred from the host material in thesinglet excited state to the guest material.

One embodiment of the present invention is a useful technique which canovercome such a problem of the efficiency of the energy transfer fromthe host material in the singlet excited state to the guest material.

Note that it has been considered so far that a light-emitting elementincluding a phosphorescent compound can theoretically achieve aninternal quantum efficiency of 100% because intersystem crossing makesit possible to convert both the single excited state and the tripletexcited state into light emission (refer to “(1) direct recombinationprocess” described above). In addition, it has been discussed that alight-emitting element having an external quantum efficiency as high as20% under the assumption that the light extraction efficiency is 20%achieved an internal quantum efficiency of substantially 100%. However,it is considered in fact that these conventional light-emitting elementshave not achieved an internal quantum efficiency of 100% because theabove-described energy transfer from the singlet excited state of thehost material has been overlooked. This is based on the fact that thepresent inventors have achieved an external quantum efficiency of 30% bycarrying out an embodiment of the present invention which is describedbelow. In other words, an external quantum efficiency of at least 30%corresponds to an internal quantum efficiency of 100%, and oneembodiment of the present invention is a useful technique for achievingit. Note that this indicates that a conventional external quantumefficiency of 20% can be estimated to correspond to an internal quantumefficiency of 70% or less.

<One Embodiment of Present Invention>

One embodiment of the present invention is a light-emitting elementwhich includes a light-emitting layer containing a phosphorescentcompound, a first organic compound, and a second organic compoundbetween a pair of electrodes, in which a combination of the firstorganic compound and the second organic compound forms an exciplex.

The first organic compound and the second organic compound form anexciplex (also referred to as excited complex) through carrierrecombination (or from a singlet exciton). In the case where theexciplex formed emits light, the emission wavelength thereof is locatedon the longer wavelength side with respect to the emission wavelength(fluorescent wavelength) of each of the first and second organiccompounds. In other words, by formation of the exciplex, the fluorescentspectrum of the first organic compound and the fluorescent spectrum ofthe second organic compound can be converted into an emission spectrumwhich is located on the longer wavelength side.

Therefore, as illustrated in FIG. 25, even when the fluorescent spectrumof the first organic compound (or the second organic compound) islocated on the shorter wavelength side with respect to the absorptionband of the phosphorescent compound which is located on the longestwavelength side, and does not have an overlap with the absorption band,an emission spectrum with a long wavelength can be obtained by formingan exciplex so as to have a large overlap with the absorption band. Thelight-emitting element of one embodiment of the present inventiontransfers energy by utilizing the overlap between the emission spectrumof the exciplex and the absorption spectrum of the phosphorescentcompound and thus has high energy transfer efficiency. Therefore, in oneembodiment of the present invention, a light-emitting element havinghigh external quantum efficiency can be obtained.

In addition, the exciplex exists only in an excited state and thus hasno ground state capable of absorbing energy. Therefore, a phenomenon inwhich the phosphorescent compound is deactivated by reverse energytransfer of the singlet excited state and triplet excited state of thephosphorescent compound to the exciplex before light emission (i.e.,emission efficiency is lowered) is not considered to occur in principle.This also contributes to improvement of external quantum efficiency.

In addition, the exciplex is considered to have an extremely smalldifference between singlet excited energy and triplet excited energy. Inother words, the emission spectrum of the exciplex from the single stateand the emission spectrum thereof from the triplet state are highlyclose to each other. Accordingly, in the case where a design isimplemented such that the emission spectrum of the exciplex (generallythe emission spectrum of the exciplex from the singlet state) overlapswith the absorption band of the phosphorescent compound on the longestwavelength side as described above, the emission spectrum of theexciplex from the triplet state (which is not observed at roomtemperature and not observed even at low temperature in many cases) alsooverlaps with the absorption band of the phosphorescent compound whichis located on the longest wavelength side. More specifically, this meansthat energy can be efficiently transferred to the phosphorescentcompound from the exciplex in both the singlet state and the tripletstate.

Molecular orbital calculations were performed as described below toverify whether or not an exciplex actually has such characteristics. Ingeneral, a combination of a heteroaromatic compound and an aromaticamine often forms an exciplex under the influence of the lowestunoccupied molecular orbital (LUMO) level of the heteroaromatic compoundwhich is deeper than the LUMO level of the aromatic amine (the propertyof easily accepting electrons) and the highest occupied molecularorbital (HOMO) level of the aromatic amine which is shallower than theHOMO level of the heteroaromatic compound (the property of easilyaccepting holes). Thus, calculations were performed using a combinationof dibenzo[f,h]quinoxaline (abbreviation: DBq) which is a typicalskeleton forming the LUMO of a heteroaromatic compound andtriphenylamine (abbreviation: TPA) which is a typical skeleton formingthe HOMO of an aromatic amine.

First, the optimal molecular structures and the excitation energies ofDBq alone and TPA alone in the lowest singlet excited state (S₁) and thelowest triplet excited state (T₁) were calculated using thetime-dependent density functional theory (TD-DFT). Furthermore, theexcitation energy of a dimer of DBq and TPA was also calculated. In theDFT, the total energy is represented as the sum of potential energy,electrostatic energy between electrons, electronic kinetic energy, andexchange-correlation energy including all the complicated interactionsbetween electrons. Also in the DFT, an exchange-correlation interactionis approximated by a functional (a function of another function) of oneelectron potential represented in terms of electron density to enablehigh-speed, high-accuracy calculations. Here, B3LYP which was a hybridfunctional was used to specify the weight of each parameter related toexchange-correlation energy. In addition, as a basis function, 6-311 (abasis function of a triple-split valence basis set using threecontraction functions for each valence orbital) was applied to all theatoms. By the above basis function, for example, 1s to 3s orbitals areconsidered in the case of hydrogen atoms, while 1s to 4s and 2p to 4porbitals are considered in the case of carbon atoms. Furthermore, toimprove calculation accuracy, the p function and the d function aspolarization basis sets were added to hydrogen atoms and atoms otherthan hydrogen atoms, respectively.

Note that Gaussian 09 was used as a quantum chemistry computationalprogram. A high performance computer (Altix 4700, manufactured by SGIJapan, Ltd.) was used for the calculations.

First, the HOMO levels and the LUMO levels of DBq alone, TPA alone, anda dimer of DBq and TPA were calculated. FIG. 64 shows the HOMO levelsand the LUMO levels, and FIGS. 65(A1), 65(A2), 65(B1), 65(B2), 65(C1),and 65(C2) show HOMO and LUMO distributions.

FIG. 65(A1) shows the LUMO distribution of DBq alone; FIG. 65(A2), theHOMO distribution of DBq alone; FIG. 65(B1), the LUMO distribution ofTPA alone; FIG. 65(B2), the HOMO distribution of TPA alone; FIG. 65(C1),the LUMO distribution of the dimer of DBq and TPA; and FIG. 65(C2), theHOMO distribution of the dimer of DBq and TPA.

As shown in FIG. 64, it is suggested that the dimer of DBq and TPA formsan exciplex of DBq and TPA under the influence of the LUMO level (−1.99eV) of DBq which is deeper (lower) than the LUMO level of TPA and theHOMO level (−5.21 eV) of TPA which is shallower (higher) than the HOMOlevel of DBq. In fact, as is clear from FIGS. 65(C1) and 65(C2), theLUMO of the dimer of DBq and TPA is distributed on the DBq side, and theHOMO thereof is distributed on the TPA side.

Next, excitation energies obtained from the optimal molecular structuresof DBq alone in S₁ and T₁ will be shown. Here, the S₁ and T₁ excitationenergies correspond to fluorescence and phosphorescence wavelengths,respectively, obtained from DBq alone. The S₁ excitation energy of DBqalone is 3.294 eV, and the fluorescence wavelength is 376.4 nm. The T₁excitation energy of DBq alone is 2.460 eV, and the phosphorescencewavelength is 504.1 nm.

In addition, excitation energies obtained from the optimal molecularstructures of TPA alone in S₁ and T₁ will be shown. Here, the S₁ and T₁excitation energies correspond to fluorescence and phosphorescencewavelengths, respectively, obtained from TPA alone. The S₁ excitationenergy of TPA alone is 3.508 eV, and the fluorescence wavelength is353.4 nm. The T₁ excitation energy of TPA alone is 2.610 eV, and thephosphorescence wavelength is 474.7 nm.

Furthermore, excitation energies obtained from the optimal molecularstructures of the dimer of DBq and TPA in S₁ and T₁ will be shown. TheS₁ and T₁ excitation energies correspond to fluorescence andphosphorescence wavelengths, respectively, obtained from the dimer ofDBq and TPA. The S₁ excitation energy of the dimer of DBq and TPA is2.036 eV, and the fluorescence wavelength is 609.1 nm. The T₁ excitationenergy of the dimer of DBq and TPA is 2.030 eV, and the phosphorescencewavelength is 610.0 nm.

It is found from the above that each of the phosphorescence wavelengthsof DBq alone and TPA alone is shifted to the longer wavelength side byabout 100 nm with respect to the fluorescence wavelength. This resultshows a tendency similar to that of CBP (measured values) describedabove and supports the validity of the calculations.

On the other hand, it is found that the fluorescence wavelength of thedimer of DBq and TPA is located on the longer wavelength side withrespect to the fluorescence wavelengths of DBq alone and TPA alone. Thisresult shows a tendency similar to that of examples (measured values)described below and supports the validity of the calculations. It isalso found that the difference between the fluorescence wavelength andthe phosphorescence wavelength of the dimer of DBq and TPA is only 0.9nm and that these wavelengths are substantially the same.

These results indicate that the exciplex can integrate the singletexcitation energy and the triplet excitation energy into substantiallythe same energy. Therefore, it is indicated as described above that theexciplex can efficiently transfer energy to the phosphorescent compoundfrom both the singlet state and the triplet state thereof.

Such an effect is specific to the use of an exciplex as a medium forenergy transfer. In general, energy transfer from the singlet excitedstate or triplet excited state of a host material to a phosphorescentcompound is considered. On the other hand, one embodiment of the presentinvention greatly differs from a conventional technique in that anexciplex of a host material and another material (an exciplex of a firstorganic compound and a second organic compound) is formed first andenergy transfer from the exciplex is used. In addition, this differenceprovides unprecedentedly high emission efficiency.

Note that in general, the use of an exciplex for a light-emitting layerof a light-emitting element has a value such as being capable ofcontrolling the emission color, but usually causes a significantdecrease in emission efficiency. Therefore, the use of an exciplex hasbeen considered unsuitable for obtaining a highly efficientlight-emitting element. However, the present inventors have found thatthe use of an exciplex as a medium for energy transfer to aphosphorescent compound enables, on the contrary, emission efficiency tobe maximized as shown in one embodiment of the present invention. Thistechnical idea conflicts with the conventional fixed idea.

To make the emission spectrum of the exciplex and the absorptionspectrum of the guest material sufficiently overlap each other, thedifference between the energy of a peak of the emission spectrum and theenergy of a peak of the absorption band on the lowest energy side in theabsorption spectrum is preferably 0.3 eV or less. The difference is morepreferably 0.2 eV or less, even more preferably 0.1 eV or less.

In addition, in one embodiment of the present invention, an exciplex isformed from a singlet exciton of the first organic compound or thesecond organic compound.

In a light-emitting element of one embodiment of the present invention,a possible elementary process of formation of an exciplex is that one ofthe first and second organic compounds forms a singlet exciton and theninteracts with the other in the ground state. As described above, theemission spectrum of the exciplex and the absorption spectrum of thephosphorescent compound can largely overlap; thus, energy transferefficiency can be increased. Accordingly, a light-emitting elementhaving high external quantum efficiency can be obtained.

The singlet exciton has a short excitation lifetime (small τ) asdescribed above. Thus, there is a problem in that part of excitationenergy is deactivated (through light emission or thermal deactivation)before the excitation energy is transferred from the singlet exciton toa guest material (Φ_(ET) tends to be small in Formula (3)). However, inone embodiment of the present invention, such deactivation of excitationenergy can be suppressed because the singlet exciton rapidly forms anexciplex. Furthermore, the exciplex has a relatively long excitationlifetime, which is considered favorable to energy transfer efficiencyΦ_(ET). Accordingly, the deactivation of the singlet excitation energyof the host material that may affect not only the efficiency of anelement but also the lifetime thereof can be suppressed by applicationof one embodiment of the present invention, so that a light-emittingelement having a long lifetime can be obtained.

In one embodiment of the present invention, it is also preferable thatthe excitation energy of the exciplex be sufficiently transferred to thephosphorescent compound, and that light emission from the exciplex benot substantially observed. Therefore, energy is preferably transferredto the phosphorescent compound through the exciplex so that thephosphorescent compound emits phosphorescence.

According to the above-described concept of energy transfer, oneembodiment of the present invention is effective in the case where atleast one of the first and second organic compounds is a fluorescentcompound (i.e., a compound which is likely to undergo light emission orthermal deactivation from the singlet excited state). Therefore, it ispreferable that at least one of the first and second organic compoundsbe a fluorescent compound.

Note that in the case where a phosphorescent compound is used as anorganic compound serving as a host material, the organic compound itselfis likely to emit light and unlikely to allow energy to be transferredto a guest material. In this case, it is favorable if the organiccompound could emit light efficiently, but it is difficult to achievehigh emission efficiency because the organic compound serving as a hostmaterial causes the problem of concentration quenching. For this reason,it is preferable that the organic compound be a fluorescent compound andenergy transfer be achieved with the above-described composition.

In addition, in one embodiment of the present invention, it ispreferable that the phosphorescent compound be an organometalliccomplex.

The exciplex used in one embodiment of the present invention will bedescribed in detail below.

<Exciplex>

The exciplex (excited complex) is formed by an interaction betweendissimilar molecules in excited states. The exciplex is generally knownto be easily formed between a material having a relatively deep LUMOlevel and a material having a relatively shallow HOMO level.

An emission wavelength depends on a difference in energy between theHOMO level and the LUMO level. When the energy difference is large, theemission wavelength is short. When the energy difference is small, theemission wavelength is long.

Here, the HOMO levels and LUMO levels of the first organic compound andthe second organic compound used in one embodiment of the presentinvention are different from each other. Specifically, the energy levelsare higher in the following order: the HOMO level of the first organiccompound <the HOMO level of the second organic compound <the LUMO levelof the first organic compound <the LUMO level of the second organiccompound (see FIG. 17).

When the exciplex is formed by these two organic compounds, the LUMOlevel and the HOMO level of the exciplex originate from the firstorganic compound and the second organic compound, respectively (see FIG.17). Therefore, the energy difference of the exciplex is smaller thanthe energy difference of the first organic compound and the energydifference of the second organic compound. In other words, the emissionwavelength of the exciplex is longer than the emission wavelengths ofthe first organic compound and the second organic compound.

The formation process of the exciplex used in one embodiment of thepresent invention is considered to be roughly classified into twoprocesses.

<<Electroplex>>

In this specification, the term “electroplex” means that the firstorganic compound in the ground state and the second organic compound inthe ground state directly form an exciplex.

As described above, in general, when an electron and a hole arerecombined in a host material, excitation energy is transferred from thehost material in an excited state to a guest material, whereby the guestmaterial is brought into an excited state to emit light.

At this time, before the excitation energy is transferred from the hostmaterial to the guest material, the host material itself emits light orthe excitation energy turns into thermal energy, which leads to partialdeactivation of the excitation energy. In particular, when the hostmaterial is in a singlet excited state, excitation lifetime is shorterthan that when it is in a triplet excited state, which easily leads todeactivation of singlet excitation energy. The deactivation ofexcitation energy is one of causes for a decrease in lifetime of alight-emitting element.

However, in one embodiment of the present invention, an electroplex isformed from the first organic compound and the second organic compoundhaving carriers (cation or anion); therefore, formation of a singletexciton having a short excitation lifetime can be suppressed. In otherwords, there can be a process where an exciplex is directly formedwithout formation of a singlet exciton. Thus, deactivation of thesinglet excitation energy can be inhibited. Accordingly, alight-emitting element having a long lifetime can be obtained.

For example, in the case where the first organic compound is a compoundhaving an electron-trapping property and the second organic compound isa compound having a hole-trapping property, an electroplex is formeddirectly from an anion of the first organic compound and a cation of thesecond organic compound. It is a novel concept to obtain alight-emitting element having high emission efficiency by suppressingthe generation of the singlet excited state of a host material andtransferring energy from an electroplex to a guest material, in theabove-described manner. Note that the generation of the triplet excitedstate of the host material is similarly suppressed and an electroplex isdirectly formed; therefore, energy transfer is considered to occur fromthe electroplex to the guest material. This mechanism is also novel.

The emission spectrum of the electroplex formed is located on the longerwavelength side with respect to the emission wavelength of each of thefirst and second organic compounds.

The overlap between the emission spectrum of the electroplex and theabsorption spectrum of the phosphorescent compound is larger than theoverlap between the emission spectrum of the first organic compound (orthe second organic compound) and the absorption spectrum of thephosphorescent compound. The light-emitting element of one embodiment ofthe present invention transfers energy by utilizing the overlap betweenthe emission spectrum of the electroplex and the absorption spectrum ofthe phosphorescent compound and thus has high energy transferefficiency. Therefore, in one embodiment of the present invention, alight-emitting element having high external quantum efficiency can beobtained.

<<Formation of Exciplex by Exciton>>

As another process, there is thought to be an elementary process whereone of the first and second organic compounds forms a singlet excitonand then interacts with the other in the ground state to form anexciplex. Unlike an electroplex, a singlet excited state of the firstorganic compound or the second organic compound is temporarily generatedin this case, but this is rapidly converted into an exciplex, and thus,deactivation of single excitation energy can be inhibited. Thus, it ispossible to inhibit deactivation of excitation energy of the firstorganic compound or the second organic compound. Accordingly, in oneembodiment of the present invention, a light-emitting element having along lifetime can be obtained. Note that it is considered that thetriplet excited state of the host material is also rapidly convertedinto an exciplex and energy is transferred from the exciplex to theguest material.

The emission spectrum of the exciplex formed is located on the longerwavelength side with respect to the emission wavelength of each of thefirst and second organic compounds.

The overlap between the emission spectrum of the exciplex and theabsorption spectrum of the phosphorescent compound is larger than theoverlap between the emission spectrum of the first organic compound (orthe second organic compound) and the absorption spectrum of thephosphorescent compound. The light-emitting element of one embodiment ofthe present invention transfers energy by utilizing the overlap betweenthe emission spectrum of the exciplex and the absorption spectrum of thephosphorescent compound and thus has high energy transfer efficiency.Accordingly, in one embodiment of the present invention, alight-emitting element having high external quantum efficiency can beobtained.

For example, in the case where the first organic compound is a compoundhaving an electron-trapping property, the second organic compound is acompound having a hole-trapping property, and the difference between theHOMO levels and the difference between the LUMO levels of thesecompounds are large (specifically, 0.3 eV or more), electrons areselectively injected into the first organic compound and holes areselectively injected into the second organic compound. In this case, itis thought that the process where an electroplex is formed takesprecedence over the process where an exciplex is formed through asinglet exciton.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 16A to 16C.

FIG. 16A illustrates a light-emitting element including an EL layer 102between a first electrode 103 and a second electrode 108. Thelight-emitting element in FIG. 16A includes a hole-injection layer 701,a hole-transport layer 702, a light-emitting layer 703, anelectron-transport layer 704, and an electron-injection layer 705 whichare stacked over the first electrode 103 in this order, and the secondelectrode 108 provided thereover.

The first electrode 103 is preferably formed using any of metals,alloys, conductive compounds, mixtures thereof, and the like which havea high work function (specifically, 4.0 eV or more). Specific examplesinclude indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tinoxide containing silicon or silicon oxide, indium oxide-zinc oxide(indium zinc oxide), indium oxide containing tungsten oxide and zincoxide (IWZO), and the like. Films of these conductive metal oxides areusually formed by a sputtering method, but may be formed by applicationof a sol-gel method or the like. For example, an indium oxide-zinc oxidefilm can be formed by a sputtering method using a target in which zincoxide is added to indium oxide at 1 wt % to 20 wt %. Further, an IWZOfilm can be formed by a sputtering method using a target in whichtungsten oxide is added to indium oxide at 0.5 wt % to 5 wt % and zincoxide is added to indium oxide at 0.1 wt % to 1 wt %. Other examples aregraphene, gold, platinum, nickel, tungsten, chromium, molybdenum, iron,cobalt, copper, palladium, nitrides of metal materials (e.g., titaniumnitride), and the like.

Note that when a layer included in the EL layer 102 and formed incontact with the first electrode 103 is formed using a later-describedcomposite material formed by combining an organic compound and anelectron acceptor (an acceptor), as a substance used for the firstelectrode 103, any of a variety of metals, alloys, electricallyconductive compounds, mixtures thereof, and the like can be usedregardless of the work function; for example, aluminum, silver, an alloycontaining aluminum (e.g., Al—Si), or the like can also be used.

The first electrode 103 can be formed by, for example, a sputteringmethod, an evaporation method (including a vacuum evaporation method),or the like.

The second electrode 108 is preferably formed using any of metals,alloys, electrically conductive compounds, mixtures thereof, and thelike which have a low work function (preferably, 3.8 eV or lower).Specific examples thereof include elements that belong to Groups 1 and 2in the periodic table, that is, alkali metals such as lithium andcesium, alkaline earth metals such as calcium and strontium, magnesium,alloys thereof (e.g., Mg—Ag and Al—Li), rare-earth metals such aseuropium and ytterbium, alloys thereof, aluminum, silver, and the like.

When a layer included in the EL layer 102 and formed in contact with thesecond electrode 108 is formed using a later-described compositematerial formed by combining an organic compound and an electron donor(a donor), any of a variety of conductive materials, such as Al, Ag,ITO, and indium oxide-tin oxide containing silicon or silicon oxide, canbe used regardless of the work function.

Note that when the second electrode 108 is formed, a vacuum evaporationmethod or a sputtering method can be used. In the case of using a silverpaste or the like, a coating method, an inkjet method, or the like canbe used.

The EL layer 102 includes at least the light-emitting layer 703. Forpart of the EL layer 102, a known substance can be used, and either alow molecular compound or a high molecular compound can be used. Notethat substances forming the EL layer 102 may consist of organiccompounds or may include an inorganic compound as a part.

Further, as illustrated in FIG. 16A, the EL layer 102 includes not onlythe light-emitting layer 703 but also an appropriate combination of thefollowing layers: the hole-injection layer 701 including a substancehaving a high hole-injection property, the hole-transport layer 702including a substance having a high hole-transport property, theelectron-transport layer 704 including a substance having a highelectron-transport property, the electron-injection layer 705 includinga substance having a high electron-injection property, and the like.

The hole-injection layer 701 is a layer that contains a substance havinga high hole-injection property. As the substance having a highhole-injection property, a metal oxide such as molybdenum oxide,titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromiumoxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide,tungsten oxide, or manganese oxide can be used. Alternatively, aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper(II) phthalocyanine (abbreviation: CuPc) can be used.

Other examples of the substance which can be used are aromatic aminecompounds and the like which are low molecular organic compounds, suchas 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Still other examples of the substance which can be used are highmolecular compounds (e.g., oligomers, dendrimers, and polymers), such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD), and high molecular compounds to which acid is added, such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),and polyaniline/poly(styrenesulfonic acid) (PAni/PSS).

For the hole-injection layer 701, the composite material formed bycombining an organic compound and an electron acceptor (an acceptor) maybe used. Such a composite material, in which holes are generated in theorganic compound by the electron acceptor, has high hole-injection andhole-transport properties. In this case, the organic compound ispreferably a material excellent in transporting the generated holes (asubstance having a high hole-transport property).

Examples of the organic compound used for the composite material can bea variety of compounds, such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, and high molecular compounds (e.g.,oligomers, dendrimers, and polymers). The organic compound used for thecomposite material is preferably an organic compound having a highhole-transport property, and is specifically preferably a substancehaving a hole mobility of 10⁻⁶ cm²/Vs or more. Note that other thanthese substances, any substance that has a property of transporting moreholes than electrons may be used. Organic compounds that can be used forthe composite material will be specifically described below.

Examples of the organic compound that can be used for the compositematerial are aromatic amine compounds, such as TDATA, MTDATA, DPAB,DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB ort-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),and carbazole derivatives, such as 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene(abbreviation: CzPA),9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Other examples of the organic compound that can be used are aromatichydrocarbon compounds, such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl] anthracene, and2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Other examples of the organic compound that can be used are aromatichydrocarbon compounds, such as2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl] anthracene(abbreviation: DPVPA).

Further, examples of the electron acceptor are organic compounds, suchas 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, oxides of transition metals, oxides of metalsthat belong to Groups 4 to 8 in the periodic table, and the like.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-accepting property.Among these, molybdenum oxide is especially preferable since it isstable in the air, has a low hygroscopic property, and is easy tohandle.

The composite material may be formed using the above-described electronacceptor and the above-described high molecular compound such as PVK,PVTPA, PTPDMA, or Poly-TPD, and may be used for the hole-injection layer701.

The hole-transport layer 702 is a layer that contains a substance havinga high hole-transport property. Examples of the substance having a highhole-transport property are aromatic amine compounds such as NPB, TPD,BPAFLP, 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). The substances mentioned here are mainlysubstances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Note thatother than these substances, any substance that has a property oftransporting more holes than electrons may be used. Note that the layercontaining a substance having a high hole-transport property is notlimited to a single layer, and may be a stack of two or more layerscontaining any of the above substances.

For the hole-transport layer 702, a carbazole derivative such as CBP,CzPA, or PCzPA or an anthracene derivative such as t-BuDNA, DNA, orDPAnth may be used.

For the hole-transport layer 702, a high molecular compound such as PVK,PVTPA, PTPDMA, or Poly-TPD can also be used.

The light-emitting layer 703 is a layer that contains a light-emittingsubstance. The light-emitting layer 703 of this embodiment contains aphosphorescent compound, a first organic compound, and a second organiccompound. The phosphorescent compound is a light-emitting substance(guest material). One of the first and second organic compounds, thecontent of which is higher than that of the other in the light-emittinglayer 703, is a host material. Embodiment 1 can be referred to forspecifics.

As the phosphorescent compound, an organometallic complex is preferable,and in particular, an iridium complex is preferable. In consideration ofenergy transfer due to Förster mechanism described above, the molarabsorption coefficient of the absorption band of the phosphorescentcompound which is located on the longest wavelength side is preferably2000 M⁻¹·cm⁻¹ or more, more preferably 5000 M⁻¹·cm⁻¹ or more. Examplesof the compound having such a high molar absorption coefficient arebis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(dpm)]),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]), and the like. In particular, when amaterial having a molar absorption coefficient of 5000 M⁻¹·cm⁻¹ or more,such as [Ir(dppm)₂(acac)], is used, a light-emitting element that canachieve an external quantum efficiency of about 30% can be obtained.

The first organic compound and the second organic compound arepreferably, for example, a combination of any one of compounds which arelikely to accept electrons (typically, heteroaromatic compounds), suchas 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTPDBq-II),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 6mDBTPDBq-II), and2-[4-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: DBTBIm-II), and any one of compounds which are likely toaccept holes (typically, aromatic amine compounds and carbazolecompounds), such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB), 4-(1-naphthyl)-4′-phenyltriphenylamine(abbreviation: αNBA1BP),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA), and 9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole(abbreviation: PCCP). With such a composition, it is possible to obtainthe effect of improvement of emission efficiency and lifetime not onlyby energy transfer from an exciplex but also by adjustment of carrierbalance between hole transport and electron transport in alight-emitting layer. Note that the present invention is not limited tothese compounds as long as the combination can form an exciplex.

Note that the exciplex may be formed at the interface between twolayers. For example, when a layer containing the second organic compoundand a layer containing the first organic compound are stacked, theexciplex is formed in the vicinity of the interface thereof. These twolayers may be used as the light-emitting layer in one embodiment of thepresent invention. In that case, the phosphorescent compound is added tothe vicinity of the interface. The phosphorescent compound may be addedto one of the two layers or both.

The electron-transport layer 704 is a layer that contains a substancehaving a high electron-transport property. Examples of the substancehaving a high electron-transport property are metal complexes such asAlq₃, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq,Zn(BOX)₂, and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂). Other examples thereof are heteroaromatic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP),4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Stillother examples are high molecular compounds such aspoly(2,5-pyridine-diyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy). The substances mentioned here are mainlysubstances having an electron mobility of 10⁻⁶ cm²/Vs or more. Note thatother than these substances, any substance that has a property oftransporting more holes than electrons may be used for theelectron-transport layer.

Further, the electron-transport layer is not limited to a single layer,and may be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 705 is a layer that contains a substancehaving a high electron-injection property. Examples of the substancethat can be used for the electron-injection layer 705 are alkali metals,alkaline earth metals, and compounds thereof, such as lithium, cesium,calcium, lithium fluoride, cesium fluoride, calcium fluoride, andlithium oxide, rare earth metal compounds, such as erbium fluoride, andthe above-mentioned substances used for the electron-transport layer704.

Alternatively, a composite material formed by combining an organiccompound and an electron donor (a donor) may be used for theelectron-injection layer 705. Such a composite material, in whichelectrons are generated in the organic compound by the electron donor,has high electron-injection and electron-transport properties. Theorganic compound here is preferably a material excellent in transportingthe generated electrons, and specifically any of the above substances(such as metal complexes and heteroaromatic compounds) for theelectron-transport layer 704 can be used. As the electron donor, asubstance showing an electron-donating property with respect to theorganic compound may be used. Preferable specific examples of theelectron donor are alkali metals, alkaline earth metals, and rare earthmetals, such as lithium, cesium, magnesium, calcium, erbium, andytterbium. Any of alkali metal oxides and alkaline earth metal oxides ispreferable, examples of which are lithium oxide, calcium oxide, bariumoxide, and the like, and a Lewis base such as magnesium oxide or anorganic compound such as tetrathiafulvalene (abbreviation: TTF) can beused.

Note that the hole-injection layer 701, the hole-transport layer 702,the light-emitting layer 703, the electron-transport layer 704, and theelectron-injection layer 705 which are mentioned above can each beformed by a method such as an evaporation method (including a vacuumevaporation method), an inkjet method, or a coating method.

A plurality of EL layers may be stacked between the first electrode 103and the second electrode 108 as illustrated in FIG. 16B. In that case, acharge-generation layer 803 is preferably provided between a first ELlayer 800 and a second EL layer 801 which are stacked. Thecharge-generation layer 803 can be formed using the above-describedcomposite material. Further, the charge-generation layer 803 may have astacked structure including a layer containing the composite materialand a layer containing another material. In that case, as the layercontaining another material, a layer containing an electron-donatingsubstance and a substance with a high electron-transport property, alayer formed of a transparent conductive film, or the like can be used.As for a light-emitting element having such a structure, problems suchas energy transfer and quenching hardly occur, and a light-emittingelement which has both high emission efficiency and a long lifetime canbe easily obtained owing to a wider choice of materials. Moreover, alight-emitting element which provides phosphorescence from one of the ELlayers and fluorescence from the other of the EL layers can be readilyobtained. This structure can be combined with any of the above-describedstructures of the EL layer.

Furthermore, by making emission colors of EL layers different, light ofa desired color can be obtained from the light-emitting element as awhole. For example, the emission colors of first and second EL layersare complementary in a light-emitting element having the two EL layers,so that the light-emitting element can be made to emit white light as awhole. Further, the same applies to a light-emitting element havingthree or more EL layers.

As illustrated in FIG. 16C, the EL layer 102 may include thehole-injection layer 701, the hole-transport layer 702, thelight-emitting layer 703, the electron-transport layer 704, anelectron-injection buffer layer 706, an electron-relay layer 707, and acomposite material layer 708 which is in contact with the secondelectrode 108, between the first electrode 103 and the second electrode108.

It is preferable to provide the composite material layer 708 which is incontact with the second electrode 108, in which case damage caused tothe EL layer 102 particularly when the second electrode 108 is formed bya sputtering method can be reduced. The composite material layer 708 canbe formed using the above-described composite material in which anorganic compound having a high hole-transport property contains anacceptor substance.

Further, by providing the electron-injection buffer layer 706, aninjection barrier between the composite material layer 708 and theelectron-transport layer 704 can be reduced; thus, electrons generatedin the composite material layer 708 can be easily injected to theelectron-transport layer 704.

For the electron-injection buffer layer 706, a substance having a highelectron-injection property, such as an alkali metal, an alkaline earthmetal, a rare earth metal, a compound of the above metal (e.g., analkali metal compound (including an oxide such as lithium oxide, ahalide, and a carbonate such as lithium carbonate or cesium carbonate),an alkaline earth metal compound (including an oxide, a halide, and acarbonate), or a rare earth metal compound (including an oxide, ahalide, and a carbonate), can be used.

Further, in the case where the electron-injection buffer layer 706contains a substance having a high electron-transport property and adonor substance, the donor substance is preferably added so that themass ratio of the donor substance to the substance having a highelectron-transport property is in the range from 0.001:1 to 0.1:1. Notethat as the donor substance, an organic compound such astetrathianaphthacene (abbreviation: TTN), nickelocene, ordecamethylnickelocene can be used as well as an alkali metal, analkaline earth metal, a rare earth metal, a compound of the above metal(e.g., an alkali metal compound (including an oxide such as lithiumoxide, a halide, and a carbonate such as lithium carbonate or cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and a carbonate), and a rare earth metal compound (including anoxide, a halide, and a carbonate). Note that as the substance having ahigh electron-transport property, a material similar to the material forthe electron-transport layer 704 described above can be used.

Furthermore, it is preferable that the electron-relay layer 707 beformed between the electron-injection buffer layer 706 and the compositematerial layer 708. The electron-relay layer 707 is not necessarilyprovided; however, by providing the electron-relay layer 707 having ahigh electron-transport property, electrons can be rapidly transportedto the electron-injection buffer layer 706.

The structure in which the electron-relay layer 707 is sandwichedbetween the composite material layer 708 and the electron-injectionbuffer layer 706 is a structure in which the acceptor substancecontained in the composite material layer 708 and the donor substancecontained in the electron-injection buffer layer 706 are less likely tointeract with each other, and thus their functions hardly interfere witheach other. Therefore, an increase in drive voltage can be prevented.

The electron-relay layer 707 contains a substance having a highelectron-transport property and is formed so that the LUMO level of thesubstance having a high electron-transport property is located betweenthe LUMO level of the acceptor substance contained in the compositematerial layer 708 and the LUMO level of the substance having a highelectron-transport property contained in the electron-transport layer704. In the case where the electron-relay layer 707 contains a donorsubstance, the donor level of the donor substance is also controlled soas to be located between the LUMO level of the acceptor substancecontained in the composite material layer 708 and the LUMO level of thesubstance having a high electron-transport property contained in theelectron-transport layer 704. As a specific value of the energy level,the LUMO level of the substance having a high electron-transportproperty contained in the electron-relay layer 707 is preferably higherthan or equal to −5.0 eV, more preferably higher than or equal to −5.0eV and lower than or equal to −3.0 eV.

As the substance having a high electron-transport property contained inthe electron-relay layer 707, a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand is preferablyused.

As the phthalocyanine-based material contained in the electron-relaylayer 707, specifically, any of CuPc, a phthalocyanine tin(II) complex(SnPc), a phthalocyanine zinc complex (ZnPc), cobalt(II) phthalocyanine,β-form (CoPc), phthalocyanine iron (FePc), and vanadyl2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (PhO-VOPc), is preferablyused.

As the metal complex having a metal-oxygen bond and an aromatic ligand,which is contained in the electron-relay layer 707, a metal complexhaving a metal-oxygen double bond is preferably used. The metal-oxygendouble bond has an acceptor property (a property of easily acceptingelectrons); thus, electrons can be transferred (donated and accepted)more easily. Further, the metal complex having a metal-oxygen doublebond is considered stable. Thus, the use of the metal complex having themetal-oxygen double bond enables the light-emitting element to be drivenmore stably at low voltage.

As the metal complex having a metal-oxygen bond and an aromatic ligand,a phthalocyanine-based material is preferable. Specifically, any ofvanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex(SnOPc), and a phthalocyanine titanium oxide complex (TiOPc) ispreferable because a metal-oxygen double bond is likely to act onanother molecule in terms of a molecular structure and an acceptorproperty is high.

Note that as the phthalocyanine-based materials mentioned above, aphthalocyanine-based material having a phenoxy group is preferable.Specifically, a phthalocyanine derivative having a phenoxy group, suchas PhO-VOPc, is preferable. The phthalocyanine derivative having aphenoxy group is soluble in a solvent and therefore has the advantage ofbeing easy to handle during formation of a light-emitting element andthe advantage of facilitating maintenance of an apparatus used for filmformation.

The electron-relay layer 707 may further contain a donor substance. Asthe donor substance, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be usedas well as an alkali metal, an alkaline earth metal, a rare earth metal,and a compound of the above metal (e.g., an alkali metal compound(including an oxide such as lithium oxide, a halide, and a carbonatesuch as lithium carbonate or cesium carbonate), an alkaline earth metalcompound (including an oxide, a halide, and a carbonate), and a rareearth metal compound (including an oxide, a halide, and a carbonate)).When such a donor substance is contained in the electron-relay layer707, electrons can be transferred easily and the light-emitting elementcan be driven at lower voltage.

In the case where a donor substance is contained in the electron-relaylayer 707, other than the materials given above as the substance havinga high electron-transport property, a substance having a LUMO levelhigher than the acceptor level of the acceptor substance contained inthe composite material layer 708 can be used. Specifically, it ispreferable to use a substance having a LUMO level higher than or equalto −5.0 eV, preferably higher than or equal to −5.0 eV and lower than orequal to −3.0 eV. As examples of such a substance, a perylenederivative, a nitrogen-containing condensed aromatic compound, and thelike are given. Note that a nitrogen-containing condensed aromaticcompound is preferably used for the electron-relay layer 707 because ofits high stability.

Specific examples of the perylene derivative are3,4,9,10-perylenetetracarboxylic dianhydride (abbreviation: PTCDA),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (abbreviation:PTCBI), N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide(abbreviation: PTCDI-C8H), N,N′-dihexyl-3,4,9,10-perylenetetracarboxylicdiimide (abbreviation: Hex PTC), and the like.

Specific examples of the nitrogen-containing condensed aromatic compoundare pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile(abbreviation: PPDN),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2PYPR),2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation: F2PYPR), andthe like.

Besides, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),1,4,5,8-naphthalenetetracarboxylic dianhydride (abbreviation: NTCDA),perfluoropentacene, copper hexadecafluorophthalocyanine (abbreviation:F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene(abbreviation: DCMT), methanofullerenes (e.g., [6,6]-phenyl C₆₁ butyricacid methyl ester), or the like can be used.

Note that in the case where a donor substance is contained in theelectron-relay layer 707, the electron-relay layer 707 may be formed bya method such as co-evaporation of the substance having a highelectron-transport property and the donor substance.

The hole-injection layer 701, the hole-transport layer 702, thelight-emitting layer 703, and the electron-transport layer 704 may eachbe formed using the above-described materials.

As described above, the EL layer 102 of this embodiment can be formed.

In the above-described light-emitting element, a current flows due to apotential difference generated between the first electrode 103 and thesecond electrode 108 and holes and electrons recombine in the EL layer102, so that light is emitted. Then, this light emission is extracted tothe outside through either the first electrode 103 or the secondelectrode 108 or both. Therefore, either the first electrode 103 or thesecond electrode 108, or both, is an electrode having a property oftransmitting visible light.

Note that the structure of layers provided between the first electrode103 and the second electrode 108 is not limited to the above-describedstructure. A structure other than the above may alternatively beemployed as long as a light-emitting region in which holes and electronsrecombine is provided in a portion away from the first electrode 103 andthe second electrode 108 so as to prevent quenching due to proximity ofthe light-emitting region to metal.

In other words, there is no particular limitation on a stack structureof the layers. A layer including a substance having a highelectron-transport property, a substance having a high hole-transportproperty, a substance having a high electron-injection property, asubstance having a high hole-injection property, a bipolar substance (asubstance having a high electron-transport property and a highhole-transport property), a hole-blocking material, or the like mayfreely be combined with a light-emitting layer.

By use of the light-emitting element described in this embodiment, apassive matrix light-emitting device or an active matrix light-emittingdevice in which driving of the light-emitting element is controlled by atransistor can be manufactured. Furthermore, the light-emitting devicecan be applied to an electronic device, a lighting device, or the like.

In the above-described manner, the light-emitting element of oneembodiment of the present invention can be manufactured.

Note that this embodiment can be combined with any of other embodimentsas appropriate.

Example 1

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 1A and 1B.

The phosphorescent compound used in this example isbis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(dpm)]). The first organic compound used inthis example is2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II). The second organic compound used in this example is4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB). Chemical formulae of the materials used in thisexample are shown below.

<Absorption Spectrum>

FIGS. 1A and 1B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(mppr-Me)₂(dpm)] that is the phosphorescent compound in adichloromethane solution of [Ir(mppr-Me)₂(dpm)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 1A and 1B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum1), an emission spectrum of a thin film of PCBNBB that is the secondorganic compound (an emission spectrum 2), and an emission spectrum of athin film of a mixed material of 2mDBTPDBq-II and PCBNBB (an emissionspectrum 3). In FIG. 1A, the horizontal axis represents wavelength (nm),and the vertical axes represent molar absorption coefficient ε(M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). In FIG. 1B, thehorizontal axis represents energy (eV), and the vertical axes representmolar absorption coefficient ε (M⁻¹·cm⁻¹) and emission intensity(arbitrary unit).

As can be seen from the absorption spectrum in FIG. 1A,[Ir(mppr-Me)₂(dpm)] has a broad absorption band at around 520 nm. Thisabsorption band is considered to greatly contribute to light emission.

The emission spectrum 3 peaks at a longer wavelength (lower energy) thanthe emission spectra 1 and 2. In addition, the peak of the emissionspectrum 3 is closer to the absorption band than the peaks of theemission spectra 1 and 2. FIGS. 1A and 1B show that the emissionspectrum 3 has the largest overlap with the absorption band in theabsorption spectrum which greatly contributes to light emission.Specifically, the difference between the peak of the absorption band inthe absorption spectrum (a shoulder peak at around 520 nm) and the peakof the emission spectrum 3 is 0.04 eV.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBNBB peaks at a longer wavelength (lower energy) thanthe emission spectrum of either organic compound alone. This indicatesthat an exciplex is formed by mixing 2mDBTPDBq-II with PCBNBB.

It is found that the emission spectrum of the mixed material has a largeoverlap with the absorption band in the absorption spectrum of[Ir(mppr-Me)₂(dpm)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(mppr-Me)₂(dpm)] and the mixed material of 2mDBTPDBq-II andPCBNBB has high energy transfer efficiency because it transfers energyby utilizing the overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

Example 2

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 2A and 2B.

The phosphorescent compound used in this example is(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]). The first organic compound used inthis example is 2mDBTPDBq-II. The second organic compound used in thisexample is PCBNBB. A chemical formula of the material used in thisexample is shown below. Note that the chemical formulae of the materialsused in Example 1 are omitted here.

<Absorption Spectrum>

FIGS. 2A and 2B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(dppm)₂(acac)] that is the phosphorescent compound in adichloromethane solution of [Ir(dppm)₂(acac)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 2A and 2B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum4), an emission spectrum of a thin film of PCNBB that is the secondorganic compound (an emission spectrum 5), and an emission spectrum of athin film of a mixed material of 2mDBTPDBq-II and PCBNBB (an emissionspectrum 6). In FIG. 2A, the horizontal axis represents wavelength (nm),and the vertical axes represent molar absorption coefficient ε(M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). In FIG. 2B, thehorizontal axis represents energy (eV), and the vertical axes representmolar absorption coefficient ε (M⁻¹·cm⁻¹) and emission intensity(arbitrary unit).

As can be seen from the absorption spectrum in FIG. 2A,[Ir(dppm)₂(acac)] has a broad absorption band at around 520 nm. Thisabsorption band is considered to greatly contribute to light emission.

The emission spectrum 6 peaks at a longer wavelength (lower energy) thanthe emission spectra 4 and 5. In addition, the peak of the emissionspectrum 6 is closer to the absorption band than the peaks of theemission spectra 4 and 5. FIGS. 2A and 2B show that the emissionspectrum 6 has the largest overlap with the absorption band in theabsorption spectrum which greatly contributes to light emission.Specifically, the difference between the peak (515 nm) of the absorptionband in the absorption spectrum and the peak of the emission spectrum 6is 0.02 eV.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBNBB peaks at a longer wavelength (lower energy) thanthe emission spectrum of either organic compound alone. This indicatesthat an exciplex is formed by mixing 2mDBTPDBq-II with PCBNBB.

It is found that the peak of the emission spectrum of the mixed materialhas a large overlap with the absorption band in the absorption spectrumof [Ir(dppm)₂(acac)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(dppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andPCBNBB has high energy transfer efficiency because it transfers energyby utilizing the overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

Example 3

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 15. Chemicalformulae of materials used in this example are shown below. Note thatthe chemical formulae of the materials used in the above examples areomitted here.

Methods for manufacturing a light-emitting element 1 of this example anda comparative light-emitting element 2 will be described below.

(Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. Note that thethickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)₂(dpm)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBNBB and [Ir(mppr-Me)₂(dpm)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:PCBNBB:[Ir(mppr-Me)₂(dpm)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film ofbathophenanthroline (abbreviation: BPhen) was formed to a thickness of20 nm to form a second electron-transport layer 1114 b.

Further, over the second electron-transport layer 1114 b, a film oflithium fluoride (LiF) was formed by evaporation to a thickness of 1 nmto form an electron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 1 of this example was fabricated.

(Comparative Light-Emitting Element 2)

A light-emitting layer 1113 of the comparative light-emitting element 2was formed by co-evaporating 2mDBTPDBq-II and [Ir(mppr-Me)₂(dpm)]. Here,the weight ratio of 2mDBTPDBq-II to [Ir(mppr-Me)₂(dpm)] was adjusted to1:0.05 (=2mDBTPDBq-II:[Ir(mppr-Me)₂(dpm)]). The thickness of thelight-emitting layer 1113 was set to 40 nm. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 1.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 1 shows element structures of the light-emitting element 1 and thecomparative light-emitting element 2 obtained as described above.

TABLE 1 first second hole- hole- electron- electron- electron- firstinjection transport transport transport injection second elecrtrodelayer layer light-emitting layer layer layer layer electrode light- ITSOBPAFLP:MoOx BPAFLP 2mDBTPDBq- 2mDBTPDBq-II BPhen LiF Al emitting 110 nm(=4:2) 20 nm II:PCBNBB:[Ir(mppr-Me)₂(dpm)] 10 nm 20 nm 1 nm 200 nmelement 1 40 nm (=0.8:0.2:0.05) 40 nm comparative ITSO BPAFLP:MoOxBPAFLP 2mDBTPDBq- 2mDBTPDBq-II BPhen LiF Al light- 110 nm (=4:2) 20 nmII:[Ir(mppr-Me)₂(dpm)] 10 nm 20 nm 1 nm 200 nm emitting 40 nm (=1:0.05)element 2 40 nm

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 3 shows current density-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.In FIG. 3, the horizontal axis represents current density (mA/cm²), andthe vertical axis represents luminance (cd/m²). FIG. 4 showsvoltage-luminance characteristics thereof. In FIG. 4, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 5 shows luminance-current efficiency characteristicsthereof. In FIG. 5, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 6 showsluminance-external quantum efficiency characteristics thereof. In FIG.6, the horizontal axis represents luminance (cd/m²), and the verticalaxis represents external quantum efficiency (%).

Further, Table 2 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 1 and the comparative light-emitting element 2 ata luminance of around 1000 cd/m².

TABLE 2 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 3.0 1.8 (0.56,0.44) 1200 64 67 24 element 1 comparative light- 3.1 2.1 (0.55, 0.44)960 45 46 17 emitting element 2

FIG. 7 shows emission spectra of the light-emitting element 1 and thecomparative light-emitting element 2 which were obtained by applying acurrent of 0.1 mA. In FIG. 7, the horizontal axis represents wavelength(nm), and the vertical axis represents emission intensity (arbitraryunit). As shown in Table 2, the CIE chromaticity coordinates of thelight-emitting element 1 at a luminance of 1200 cd/m² were (x, y)=(0.56,0.44), and the CIE chromaticity coordinates of the comparativelight-emitting element 2 at a luminance of 960 cd/m² were (x, y)=(0.55,0.44). These results show that orange light emission originating from[Ir(mppr-Me)₂(dpm)] was obtained from the light-emitting element 1 andthe comparative light-emitting element 2.

As can be seen from Table 2 and FIGS. 3 to 6, the light-emitting element1 has higher current efficiency, higher power efficiency, and higherexternal quantum efficiency than the comparative light-emitting element2.

In the light-emitting element 1 of this example, 2mDBTPDBq-II, PCBNBB,and [Ir(mppr-Me)₂(dpm)] described in Example 1 are used for thelight-emitting layer. As described in Example 1, the emission spectrumof the mixed material of 2mDBTPDBq-II and PCBNBB (the emission spectrumof an exciplex) has a larger overlap with the absorption spectrum of[Ir(mppr-Me)₂(dpm)] as compared with the emission spectrum of2mDBTPDBq-II or PCBNBB alone. The light-emitting element 1 of thisexample is considered to have high energy transfer efficiency because ittransfers energy by utilizing the overlap, and therefore have higherexternal quantum efficiency than the comparative light-emitting element2.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 1 and the comparative light-emittingelement 2 were subjected to reliability tests. Results of thereliability tests are shown in FIG. 8. In FIG. 8, the vertical axisrepresents normalized luminance (%) with an initial luminance of 100%,and the horizontal axis represents driving time (h) of the elements.

In the reliability tests, the light-emitting element 1 and thecomparative light-emitting element 2 were driven under the conditionswhere the initial luminance was set to 5000 cd/m² and the currentdensity was constant.

The luminance of the comparative light-emitting element 2 after 120hours was 58% of the initial luminance. The luminance of thelight-emitting element 1 after 630 hours was 65% of the initialluminance. From the results, the light-emitting element 1 is found tohave a longer lifetime than the comparative light-emitting element 2.

The above results show that an element having high reliability can beobtained by application of one embodiment of the present invention.

Example 4

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

A method for manufacturing a light-emitting element 3 of this examplewill be described below.

(Light-Emitting Element 3)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBNBB, and [Ir(dppm)₂(acac)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBNBB and [Ir(dppm)₂(acac)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:PCBNBB:[Ir(dppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 20 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 3 of this example was fabricated.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 3 shows an element structure of the light-emitting element 3obtained as described above.

TABLE 3 first second hole- hole- electron- electron- electron- firstinjection transport transport transport injection second elecrtrodelayer layer light-emitting layer layer layer layer electrode light- ITSOBPAFLP:MoOx BPAFLP 2mDBTPDBq- 2mDBTPDBq-II BPhen LiF Al emitting 110 nm(=4:2) 20 nm II:PCBNBB:[Ir(dppm)₂(acac)] 10 nm 20 nm 1 nm 200 nm element3 40 nm (=0.8:0.2:0.05) 40 nm

In a glove box containing a nitrogen atmosphere, the light-emittingelement 3 was sealed so as not to be exposed to air. Then, operationcharacteristics of the light-emitting element were measured. Note thatthe measurements were carried out at room temperature (in the atmospherekept at 25° C.).

FIG. 9 shows current density-luminance characteristics of thelight-emitting element 3. In FIG. 9, the horizontal axis representscurrent density (mA/cm²), and the vertical axis represents luminance(cd/m²). FIG. 10 shows voltage-luminance characteristics thereof. InFIG. 10, the horizontal axis represents voltage (V), and the verticalaxis represents luminance (cd/m²). FIG. 11 shows luminance-currentefficiency characteristics thereof. In FIG. 11, the horizontal axisrepresents luminance (cd/m²), and the vertical axis represents currentefficiency (cd/A). FIG. 12 shows luminance-external quantum efficiencycharacteristics thereof. In FIG. 12, the horizontal axis representsluminance (cd/m²), and the vertical axis represents external quantumefficiency (%).

Further, Table 4 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of thelight-emitting element 3 at a luminance of 1100 cd/m².

TABLE 4 external current chromaticity current power quantum voltagedensity coordinates efficiency efficiency efficiency (V) (mA/cm²) (x, y)(cd/A) (lm/W) (%) light-emitting 2.9 1.5 (0.54, 0.46) 77 83 28 element 3

FIG. 13 shows an emission spectrum of the light-emitting element 3 whichwas obtained by applying a current of 0.1 mA. In FIG. 13, the horizontalaxis represents wavelength (nm), and the vertical axis representsemission intensity (arbitrary unit). As shown in Table 4, the CIEchromaticity coordinates of the light-emitting element 3 at a luminanceof 1100 cd/m² were (x, y)=(0.54, 0.46). These results show that orangelight emission originating from [Ir(dppm)₂(acac)] was obtained from thelight-emitting element 3.

As can be seen from Table 4 and FIGS. 9 to 12, the light-emittingelement 3 has high current efficiency, high power efficiency, and highexternal quantum efficiency. In particular, the light-emitting element 3has an extremely high external quantum efficiency of 28% at a luminanceof 1100 cd/m². As described above, the upper limit of external quantumefficiency is said to be around 25%. However, the result of the externalquantum efficiency this time is above the limit.

In the light-emitting element of this example, 2mDBTPDBq-II, PCBNBB, and[Ir(dppm)₂(acac)] described in Example 2 are used for the light-emittinglayer. As described in Example 2, the emission spectrum of the mixedmaterial of 2mDBTPDBq-II and PCBNBB (the emission spectrum of anexciplex) has a larger overlap with the absorption spectrum of[Ir(dppm)₂(acac)] as compared with the emission spectrum of 2mDBTPDBq-IIor PCBNBB alone. The light-emitting element of this example isconsidered to have high energy transfer efficiency because it transfersenergy by utilizing the overlap, and therefore have unprecedentedly highexternal quantum efficiency.

The results of Example 2 show that a peak of the absorption band on thelongest wavelength side in the absorption spectrum of the guest materialused in the light-emitting element 3 is close to a peak of the emissionspectrum, and that the molar absorption coefficient at the peak is high(>5000 M⁻¹·cm⁻¹). From these results, the light-emitting element 3 isconsidered to have particularly high energy transfer efficiency andtherefore have unprecedentedly high external quantum efficiency.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 3 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 14. In FIG. 14, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the elements.

In the reliability tests, the light-emitting element 3 was driven underthe conditions where the initial luminance was set to 5000 cd/m² and thecurrent density was constant.

The light-emitting element 3 kept 92% of the initial luminance after thedriving for 320 hours.

The above results show that an element having high reliability can beobtained by application of one embodiment of the present invention.

Example 5

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 18A and 18B.

The phosphorescent compound used in this example is [Ir(dppm)₂(acac)].The first organic compound used in this example is 2mDBTPDBq-II. Thesecond organic compound used in this example is4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP). A chemical formula of the material used in this example isshown below. Note that the chemical formulae of the materials used inthe above examples are omitted here.

<Absorption Spectrum>

FIGS. 18A and 18B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(dppm)₂(acac)] that is the phosphorescent compound in adichloromethane solution of [Ir(dppm)₂(acac)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 18A and 18B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum7), an emission spectrum of a thin film of PCBA1BP that is the secondorganic compound (an emission spectrum 8), and an emission spectrum of athin film of a mixed material of 2mDBTPDBq-II and PCBA1BP (an emissionspectrum 9). In FIG. 18A, the horizontal axis represents wavelength(nm), and the vertical axes represent molar absorption coefficient ε(M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). In FIG. 18B, thehorizontal axis represents energy (eV), and the vertical axes representmolar absorption coefficient ε (M⁻¹·cm⁻¹) and emission intensity(arbitrary unit).

As can be seen from the absorption spectrum in FIG. 18A,[Ir(dppm)₂(acac)] has a broad absorption band at around 520 nm. Thisabsorption band is considered to greatly contribute to light emission.

The emission spectrum 9 peaks at a longer wavelength (lower energy) thanthe emission spectra 7 and 8. In addition, the peak of the emissionspectrum 9 is closer to the absorption band than the peaks of theemission spectra 7 and 8. FIGS. 18A and 18B show that the emissionspectrum 9 has the largest overlap with the absorption band in theabsorption spectrum which greatly contributes to light emission.Specifically, the difference between the peak (515 nm) of the absorptionband in the absorption spectrum and the peak of the emission spectrum 9is 0.02 eV.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBA1BP peaks at a longer wavelength (lower energy)than the emission spectrum of either organic compound alone. Thisindicates that an exciplex is formed by mixing 2mDBTPDBq-II withPCBA1BP.

It is found that the peak of the emission spectrum of the mixed materialhas a large overlap with the absorption band in the absorption spectrumof [Ir(dppm)₂(acac)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(dppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andPCBA1BP has high energy transfer efficiency because it transfers energyby utilizing the overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

Example 6

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

A method for manufacturing a light-emitting element 4 of this examplewill be described below.

(Light-Emitting Element 4)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(dppm)₂(acac)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBA1BP and [Ir(dppm)₂(acac)] was adjusted to 0.8:0.2:0.1(=2mDBTPDBq-II:PCBA1BP:[Ir(dppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 15 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 15 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 4 of this example was fabricated.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 5 shows an element structure of the light-emitting element 4obtained as described above.

TABLE 5 first second hole- electron- electron- electron- second firsthole-injection transport transport transport injection elec- elecrtrodelayer layer light-emitting layer layer layer layer trode light- ITSOBPAFLP:MoOx BPAFLP 2mDBTPDBq-II:PCBA1BP:[Ir(dppm)₂(acac)] 2mDBTPDBq-BPhen LiF Al emitting 110 nm (=4:2) 20 nm (=0.8:0.2:0.1) II 15 nm 1 nm200 nm element 40 nm 40 nm 15 nm 4

In a glove box containing a nitrogen atmosphere, the light-emittingelement 4 was sealed so as not to be exposed to air. Then, operationcharacteristics of the light-emitting element were measured. Note thatthe measurements were carried out at room temperature (in the atmospherekept at 25° C.).

FIG. 19 shows current density-luminance characteristics of thelight-emitting element 4. In FIG. 19, the horizontal axis representscurrent density (mA/cm²), and the vertical axis represents luminance(cd/m²). FIG. 20 shows voltage-luminance characteristics thereof. InFIG. 20, the horizontal axis represents voltage (V), and the verticalaxis represents luminance (cd/m²). FIG. 21 shows luminance-currentefficiency characteristics thereof. In FIG. 21, the horizontal axisrepresents luminance (cd/m²), and the vertical axis represents currentefficiency (cd/A). FIG. 22 shows luminance-external quantum efficiencycharacteristics thereof. In FIG. 22, the horizontal axis representsluminance (cd/m²), and the vertical axis represents external quantumefficiency (%).

Further, Table 6 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of thelight-emitting element 4 at a luminance of 1100 cd/m².

TABLE 6 external current chromaticity current power quantum voltagedensity coordinates efficiency efficiency efficiency (V) (mA/cm²) (x, y)(cd/A) (lm/W) (%) light-emitting 3.0 1.4 (0.57, 0.43) 76 70 31 element 4

FIG. 23 shows an emission spectrum of the light-emitting element 4 whichwas obtained by applying a current of 0.1 mA. In FIG. 23, the horizontalaxis represents wavelength (nm), and the vertical axis representsemission intensity (arbitrary unit). As shown in Table 6, the CIEchromaticity coordinates of the light-emitting element 4 at a luminanceof 1100 cd/m² were (x, y)=(0.57, 0.43). These results show that orangelight emission originating from [Ir(dppm)₂(acac)] was obtained from thelight-emitting element 4.

As can be seen from Table 6 and FIGS. 19 to 22, the light-emittingelement 4 has high current efficiency, high power efficiency, and highexternal quantum efficiency. In particular, the light-emitting element 4has an extremely high external quantum efficiency of 31% at a luminanceof 1100 cd/m². As described above, the upper limit of external quantumefficiency is said to be around 25%. However, the result of the externalquantum efficiency this time is above the limit.

In the light-emitting element of this example, 2mDBTPDBq-II, PCBA1BP,and [Ir(dppm)₂(acac)] described in Example 5 are used for thelight-emitting layer. As described in Example 5, the emission spectrumof the mixed material of 2mDBTPDBq-II and PCBA1BP (the emission spectrumof an exciplex) has a larger overlap with the absorption spectrum of[Ir(dppm)₂(acac)] as compared with the emission spectrum of 2mDBTPDBq-IIor PCBA1BP alone. The light-emitting element of this example isconsidered to have high energy transfer efficiency because it transfersenergy by utilizing the overlap, and therefore have unprecedentedly highexternal quantum efficiency.

The results of Example 5 show that a peak of the absorption band on thelongest wavelength side in the absorption spectrum of the guest materialused in the light-emitting element 4 is close to a peak of the emissionspectrum, and that the molar absorption coefficient at the peak is high(>5000 M⁻¹·cm⁻¹). From these results, the light-emitting element 4 isconsidered to have particularly high energy transfer efficiency andtherefore have unprecedentedly high external quantum efficiency.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 4 was subjected to reliability tests.Results of the reliability tests are shown in FIG. 24. In FIG. 24, thevertical axis represents normalized luminance (%) with an initialluminance of 100%, and the horizontal axis represents driving time (h)of the elements.

In the reliability tests, the light-emitting element 4 was driven underthe conditions where the initial luminance was set to 5000 cd/m² and thecurrent density was constant.

The light-emitting element 4 kept 95% of the initial luminance after thedriving for 170 hours.

The above results show that an element having high reliability can beobtained by application of one embodiment of the present invention.

Example 7

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 26A and 26B.

The phosphorescent compound used in this example is [Ir(dppm)₂(acac)].The first organic compound used in this example is 2mDBTPDBq-II. Thesecond organic compound used in this example is4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB). Achemical formula of the material used in this example is shown below.Note that the chemical formulae of the materials used in the aboveexamples are omitted here.

<Absorption Spectrum>

FIGS. 26A and 26B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(dppm)₂(acac)] that is the phosphorescent compound in adichloromethane solution of [Ir(dppm)₂(acac)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 26A and 26B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum10) and an emission spectrum of a thin film of a mixed material of2mDBTPDBq-II and NPB (an emission spectrum 11). In FIG. 26A, thehorizontal axis represents wavelength (nm), and the vertical axesrepresent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit). In FIG. 26B, the horizontal axis representsenergy (eV), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 26A,[Ir(dppm)₂(acac)] has a broad absorption band at around 520 nm. Thisabsorption band is considered to greatly contribute to light emission.

The emission spectrum 11 peaks at a longer wavelength (lower energy)than the emission spectrum 10. In addition, the peak of the emissionspectrum 11 is closer to the absorption band than the peak of theemission spectrum 10. It is known that the emission spectrum of NPB thatis the second organic compound peaks at around 430 nm. This means thatthe emission spectrum 11 peaks at a longer wavelength (lower energy)than the emission spectrum of NPB. It also means that the peak of theemission spectrum 11 is closer to the absorption band than the peak ofthe emission spectrum of NPB. The above findings show that the emissionspectrum 11 has the largest overlap with the absorption band in theabsorption spectrum which greatly contributes to light emission.Specifically, the difference between the peak (515 nm) of the absorptionband in the absorption spectrum and the peak of the emission spectrum 11is 0.09 eV.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and NPB peaks at a longer wavelength (lower energy) thanthe emission spectrum of either organic compound alone. This indicatesthat an exciplex is formed by mixing 2mDBTPDBq-II with NPB.

It is found that the peak of the emission spectrum of the mixed materialhas a large overlap with the absorption band in the absorption spectrumof [Ir(dppm)₂(acac)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(dppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andNPB has high energy transfer efficiency because it transfers energy byutilizing the overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

Example 8

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 15. Achemical formula of a material used in this example is shown below. Notethat structural formulae of the materials used in the above examples areomitted here.

Methods for manufacturing a light-emitting element 5 of this example anda comparative light-emitting element 6 will be described below.

(Light-Emitting Element 5)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 4,4′,4″-(1,3,5-benzenetriyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2(=DBT3P-II:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, NPB, and [Ir(dppm)₂(acac)] were co-evaporatedto form a light-emitting layer 1113 over the hole-transport layer 1112.Here, the weight ratio of 2mDBTPDBq-II to NPB and [Ir(dppm)₂(acac)] wasadjusted to 0.8:0.2:0.05 (=2mDBTPDBq-II:NPB:[Ir(dppm)₂(acac)]). Thethickness of the light-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 20 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 5 of this example was fabricated.

(Comparative Light-Emitting Element 6)

A light-emitting layer 1113 of the comparative light-emitting element 6was formed by co-evaporating 2mDBTPDBq-II and [Ir(dppm)₂(acac)]. Here,the weight ratio of 2mDBTPDBq-II to [Ir(dppm)₂(acac)] was adjusted to1:0.05 (=2mDBTPDBq-II:[Ir(dppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 5.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 7 shows element structures of the light-emitting element 5 and thecomparative light-emitting element 6 obtained as described above.

TABLE 7 second hole- electron- electron- second first hole-injectiontransport first electron- transport injection elec- elecrtrode layerlayer light-emitting layer transport layer layer layer trode light- ITSODBT3P-II:MoOx BPAFLP 2mDBTPDBq-II:NPB:[Ir(dppm)₂(acac)] 2mDBTPDBq-IIBPhen LiF Al emitting 110 nm (=4:2) 20 nm (=0.8:0.2:0.05) 10 nm 20 nm 1nm 200 nm element 5 40 nm 40 nm comparative ITSO DBT3P-II:MoOx BPAFLP2mDBTPDBq-II:[Ir(dppm)₂(acac)] 2mDBTPDBq-II BPhen LiF Al light- 110 nm(=4:2) 20 nm (=1:0.05) 10 nm 20 nm 1 nm 200 nm emitting 40 nm 40 nmelement 6

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 27 shows current density-luminance characteristics of thelight-emitting element 5 and the comparative light-emitting element 6.In FIG. 27, the horizontal axis represents current density (mA/cm²), andthe vertical axis represents luminance (cd/m²). FIG. 28 showsvoltage-luminance characteristics thereof. In FIG. 28, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 29 shows luminance-current efficiency characteristicsthereof. In FIG. 29, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 30shows luminance-external quantum efficiency characteristics thereof. InFIG. 30, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 8 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 5 and the comparative light-emitting element 6 ata luminance of around 1000 cd/m².

TABLE 8 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 2.9 1.5 (0.57,0.43) 1100 75 81 29 element 5 comparative light- 2.9 1.3 (0.56, 0.44)830 65 71 24 emitting element 6

FIG. 31 shows emission spectra of the light-emitting element 5 and thecomparative light-emitting element 6 which were obtained by applying acurrent of 0.1 mA. In FIG. 31, the horizontal axis represents wavelength(nm), and the vertical axis represents emission intensity (arbitraryunit). As shown in Table 8, the CIE chromaticity coordinates of thelight-emitting element 5 at a luminance of 1100 cd/m² were (x, y)=(0.57,0.43), and the CIE chromaticity coordinates of the comparativelight-emitting element 6 at a luminance of 830 cd/m² were (x, y)=(0.56,0.44). These results show that orange light emission originating from[Ir(dppm)₂(acac)] was obtained from the light-emitting element 5 and thecomparative light-emitting element 6.

As can be seen from Table 8 and FIGS. 27 to 30, the light-emittingelement 5 has higher current efficiency, higher power efficiency, andhigher external quantum efficiency than the comparative light-emittingelement 6.

In the light-emitting element 5, 2mDBTPDBq-II, NPB, and[Ir(dppm)₂(acac)] described in Example 7 are used for the light-emittinglayer. As described in Example 7, the emission spectrum of the mixedmaterial of 2mDBTPDBq-II and NPB (the emission spectrum of an exciplex)has a larger overlap with the absorption band in the absorption spectrumof [Ir(dppm)₂(acac)] which is considered to greatly contribute to lightemission, as compared with the emission spectrum of 2mDBTPDBq-II alone.The light-emitting element 5 is considered to have high energy transferefficiency because it transfers energy by utilizing the overlap, andtherefore have higher external quantum efficiency than the comparativelight-emitting element 6.

The results of Example 7 show that a peak of the absorption band on thelongest wavelength side in the absorption spectrum of the phosphorescentcompound used in the light-emitting element 5 is close to a peak of theemission spectrum, and that the molar absorption coefficient at the peakis high (>5000 M⁻¹·cm⁻¹). From these results, the light-emitting element5 is considered to have particularly high energy transfer efficiency andtherefore have unprecedentedly high external quantum efficiency.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Example 9

In this example, an example of a combination of a first organiccompound, a second organic compound, and a phosphorescent compound whichcan be used for a light-emitting element of one embodiment of thepresent invention will be described with reference to FIGS. 32A and 32B.

The phosphorescent compound used in this example isbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]). The first organic compound used inthis example is 2mDBTPDBq-II. The second organic compound used in thisexample is NPB. A chemical formula of the material used in this exampleis shown below. Note that the chemical formulae of the materials used inthe above examples are omitted here.

<Absorption Spectrum>

FIGS. 32A and 32B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(tppr)₂(dpm)] that is the phosphorescent compound in adichloromethane solution of [Ir(tppr)₂(dpm)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.094 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 32A and 32B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum12) and an emission spectrum of a thin film of a mixed material of2mDBTPDBq-II and NPB (an emission spectrum 13). In FIG. 32A, thehorizontal axis represents wavelength (nm), and the vertical axesrepresent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit). In FIG. 32B, the horizontal axis representsenergy (eV), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 32A,[Ir(tppr)₂(dpm)] has a broad absorption band at around 530 nm. Thisabsorption band is considered to greatly contribute to light emission.

The emission spectrum 13 peaks at a longer wavelength (lower energy)than the emission spectrum 12. In addition, the peak of the emissionspectrum 13 is closer to the absorption band than the peak of theemission spectrum 12. It is known that the emission spectrum of NPB thatis the second organic compound peaks at around 430 nm. This means thatthe emission spectrum 13 peaks at a longer wavelength (lower energy)than the emission spectrum of NPB. It also means that the peak of theemission spectrum 13 is closer to the absorption band than the peak ofthe emission spectrum of NPB. The above findings show that the emissionspectrum 13 has the largest overlap with the absorption band in theabsorption spectrum which greatly contributes to light emission.Specifically, the difference between the peak of the absorption band inthe absorption spectrum (a shoulder peak at around 530 nm) and the peakof the emission spectrum 13 is 0.01 eV.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and NPB peaks at a longer wavelength (lower energy) thanthe emission spectrum of either organic compound alone. This indicatesthat an exciplex is formed by mixing 2mDBTPDBq-II with NPB.

It is found that the peak of the emission spectrum of the mixed materialhas a large overlap with the absorption band in the absorption spectrumof [Ir(tppr)₂(dpm)] which is considered to greatly contribute to lightemission. Thus, it is indicated that the light-emitting elementincluding [Ir(tppr)₂(dpm)] and the mixed material of 2mDBTPDBq-II andNPB has high energy transfer efficiency because it transfers energy byutilizing the overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

Example 10

In this example, a light-emitting element of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

Methods for manufacturing a light-emitting element 7 of this example anda comparative light-emitting element 8 will be described below.

(Light-Emitting Element 7)

A light-emitting layer 1113 of the light-emitting element 7 was formedby co-evaporating 2mDBTPDBq-II, NPB, and [Ir(tppr)₂(dpm)]. Here, theweight ratio of 2mDBTPDBq-II to NPB and [Ir(tppr)₂(dpm)] was adjusted to0.8:0.2:0.05 (=2mDBTPDBq-II:NPB:[Ir(tppr)₂(dpm)]). The thickness of thelight-emitting layer 1113 was set to 40 nm. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 5 described in Example 8.

(Comparative Light-Emitting Element 8)

A light-emitting layer 1113 of the comparative light-emitting element 8was formed by co-evaporating 2mDBTPDBq-II and [Ir(tppr)₂(dpm)]. Here,the weight ratio of 2mDBTPDBq-II to [Ir(tppr)₂(dpm)] was adjusted to1:0.05 (=2mDBTPDBq-II: [Ir(tppr)₂(dpm)]. The thickness of thelight-emitting layer 1113 was set to 40 nm. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 5 described in Example 8.

Table 9 shows element structures of the light-emitting element 7 and thecomparative light-emitting element 8 obtained as described above.

TABLE 9 second hole- electron- electron- second first hole-injectiontransport first electron- transport injection elec- elecrtrode layerlayer light-emitting layer transport layer layer layer trode light- ITSODBT3P-II:MoOx BPAFLP 2mDBTPDBq-II:NPB:[Ir(tppr)₂(dpm)] 2mDBTPDBq-IIBPhen LiF Al emitting 110 nm (=4:2) 20 nm (=0.8:0.2:0.05) 10 nm 20 nm 1nm 200 nm element 7 40 nm 40 nm comparative ITSO DBT3P-II:MoOx BPAFLP2mDBTPDBq-II:[Ir(tppr)₂(dpm)] 2mDBTPDBq-II BPhen LiF Al light- 110 nm(=4:2) 20 nm (=1:0.05) 10 nm 20 nm 1 nm 200 nm emitting 40 nm 40 nmelement 8

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 33 shows current density-luminance characteristics of thelight-emitting element 7 and the comparative light-emitting element 8.In FIG. 33, the horizontal axis represents current density (mA/cm²), andthe vertical axis represents luminance (cd/m²). FIG. 34 showsvoltage-luminance characteristics thereof. In FIG. 34, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 35 shows luminance-current efficiency characteristicsthereof. In FIG. 35, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 36shows luminance-external quantum efficiency characteristics thereof. InFIG. 36, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 10 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 7 and the comparative light-emitting element 8 ata luminance of around 1000 cd/m².

TABLE 10 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 3.3 4.1 (0.66,0.34) 1100 26 25 22 element 7 comparative light- 3.6 7.4 (0.66, 0.34)1000 14 12 11 emitting element 8

FIG. 37 shows emission spectra of the light-emitting element 7 and thecomparative light-emitting element 8 which were obtained by applying acurrent of 0.1 mA. In FIG. 37, the horizontal axis represents wavelength(nm), and the vertical axis represents emission intensity (arbitraryunit). As shown in Table 10, the CIE chromaticity coordinates of thelight-emitting element 7 at a luminance of 1100 cd/m² were (x, y)=(0.66,0.34), and the CIE chromaticity coordinates of the comparativelight-emitting element 8 at a luminance of 1000 cd/m² were (x, y)=(0.66,0.34). These results show that red light emission originating from[Ir(tppr)₂(dpm)] was obtained from the light-emitting element 7 and thecomparative light-emitting element 8.

As can be seen from Table 10 and FIGS. 33 to 36, the light-emittingelement 7 has higher current efficiency, higher power efficiency, andhigher external quantum efficiency than the comparative light-emittingelement 8.

In the light-emitting element 7, 2mDBTPDBq-II, NPB, and [Ir(tppr)₂(dpm)]described in Example 9 are used for the light-emitting layer. Asdescribed in Example 9, the emission spectrum of the mixed material of2mDBTPDBq-II and NPB (the emission spectrum of an exciplex) has a largeroverlap with the absorption band in the absorption spectrum of[Ir(tppr)₂(dpm)] which is considered to greatly contribute to lightemission, as compared with the emission spectrum of 2mDBTPDBq-II alone.The light-emitting element 7 is considered to have high energy transferefficiency because it transfers energy by utilizing the overlap, andtherefore have higher external quantum efficiency than the comparativelight-emitting element 8.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 7 and the comparative light-emittingelement 8 were subjected to reliability tests. Results of thereliability tests are shown in FIG. 38. In FIG. 38, the vertical axisrepresents normalized luminance (%) with an initial luminance of 100%,and the horizontal axis represents driving time (h) of the elements.

In the reliability tests, the light-emitting element 7 and thecomparative light-emitting element 8 were driven under the conditionswhere the initial luminance was set to 5000 cd/m² and the currentdensity was constant.

The luminance of the comparative light-emitting element 8 after 97 hourswas 63% of the initial luminance. The luminance of the light-emittingelement 7 after 98 hours was 87% of the initial luminance. From theresults, the light-emitting element 7 is found to have a longer lifetimethan the comparative light-emitting element 8.

The above results show that an element having high reliability can beobtained by application of one embodiment of the present invention.

Example 11

In this example, examples of combinations of a first organic compound, asecond organic compound, and a phosphorescent compound which can be usedfor a light-emitting element of one embodiment of the present inventionwill be described with reference to FIGS. 39A and 39B.

The phosphorescent compound used in this example is(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]). The first organic compound used inthis example is 2mDBTPDBq-II. Two kinds of second organic compounds,PCBA1BP and 4-(1-naphthyl)-4′-phenyltriphenylamine (abbreviation:αNBA1BP), are used in this example. Chemical formulae of the materialsused in this example are shown below. Note that the chemical formulae ofthe materials used in the above examples are omitted here.

<Absorption Spectrum>

FIGS. 39A and 39B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(mppm)₂(acac)] that is the phosphorescent compound in adichloromethane solution of [Ir(mppm)₂(acac)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.10 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 39A and 39B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum14), an emission spectrum of a thin film of PCBA1BP that is the secondorganic compound (an emission spectrum 15), an emission spectrum of athin film of αNBA1BP that is the second organic compound (an emissionspectrum 16), an emission spectrum of a thin film of a mixed material of2mDBTPDBq-II and PCBA1BP (an emission spectrum 17), and an emissionspectrum of a thin film of a mixed material of 2mDBTPDBq-II and αNBA1BP(an emission spectrum 18). In FIG. 39A, the horizontal axis representswavelength (nm), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). InFIG. 39B, the horizontal axis represents energy (eV), and the verticalaxes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 39A,[Ir(mppm)₂(acac)] has a broad absorption band at around 490 nm. Thisabsorption band is considered to greatly contribute to light emission.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBA1BP (the emission spectrum 17) and the emissionspectrum of the mixed material of 2mDBTPDBq-II and αNBA1BP (the emissionspectrum 18) each peak at a longer wavelength (lower energy) than theemission spectrum of either organic compound alone. This indicates thatan exciplex is formed by mixing 2mDBTPDBq-II with PCBA1BP. It alsoindicates that an exciplex is formed by mixing 2mDBTPDBq-II withαNBA1BP.

It is found that the peak of the emission spectrum of each mixedmaterial has a large overlap with the absorption band in the absorptionspectrum of [Ir(mppm)₂(acac)] which is considered to greatly contributeto light emission. Thus, it is indicated that the light-emitting elementincluding [Ir(mppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andPCBA1BP and the light-emitting element including [Ir(mppm)₂(acac)] andthe mixed material of 2mDBTPDBq-II and αNBA1BP have high energy transferefficiency because they each transfer energy by utilizing the overlapbetween the emission spectrum of the mixed material and the absorptionspectrum of the phosphorescent compound. Accordingly, it is indicatedthat a light-emitting element having high external quantum efficiencycan be obtained.

The emission spectrum 18 peaks at a shorter wavelength (higher energy)than the emission spectrum 17. In addition, the peak of the emissionspectrum 18 is closer to the absorption band than the peak of theemission spectrum 17. Specifically, the difference between the peak ofthe absorption band in the absorption spectrum (a shoulder peak ataround 490 nm) and the peak of the emission spectrum 17 is 0.15 eV, andthe difference between the peak of the absorption band in the absorptionspectrum (a shoulder peak at around 490 nm) and the peak of the emissionspectrum 18 is 0.01 eV.

The difference between the peaks of the emission spectrum 17 and theemission spectrum 18 is considered to be due to the difference betweenthe HOMO levels of PCBA1BP and cNBA1BP. Specifically, the HOMO level ofPCBA1BP is −5.43 eV, whereas the HOMO level of αNBA1BP is −5.52 eV (eachcalculated by cyclic voltammetry (CV) measurement). Because αNBA1BP hasa lower (deeper) HOMO level than PCBA1BP, the emission spectrum 18 isconsidered to peak at a shorter wavelength (higher energy) than theemission spectrum 17.

Example 12

In this example, light-emitting elements of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

Methods for manufacturing a light-emitting element 9 and alight-emitting element 10 of this example will be described below.

(Light-Emitting Element 9)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBA1BP, and [Ir(mppm)₂(acac)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBA1BP and [Ir(mppm)₂(acac)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:PCBA1BP:[Ir(mppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 20 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 9 of this example was fabricated.

(Light-Emitting Element 10)

A light-emitting layer 1113 of the light-emitting element 10 was formedby co-evaporating 2mDBTPDBq-II, αNBA1BP, and [Ir(mppm)₂(acac)]. Here,the weight ratio of 2mDBTPDBq-II to αNBA1BP and [Ir(mppm)₂(acac)] wasadjusted to 0.8:0.2:0.05 (=2mDBTPDBq-II:αNBA1BP:[Ir(mppm)₂(acac)]). Thethickness of the light-emitting layer 1113 was set to 40 nm. Componentsother than the light-emitting layer 1113 were manufactured in a mannersimilar to that of the light-emitting element 9.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 11 shows element structures of the light-emitting element 9 andthe light-emitting element 10 obtained as described above.

TABLE 11 first second hole- electron- electron- electron- second firsthole-injection transport transport transport injection elec- elecrtrodelayer layer light-emitting layer layer layer layer trode light- ITSOBPAFLP:MoOx BPAFLP 2mDBTPDBq-II:PCBA1BP:[Ir(mppm)₂(acac)] 2mDBTPDBq-BPhen LiF Al emitting 110 nm (=4:2) 20 nm (=0.8:0.2:0.05) II 20 nm 1 nm200 nm element 40 nm 40 nm 10 nm 9 light- ITSO BPAFLP:MoOx BPAFLP2mDBTPDBq-II:αNBA1BP:[Ir(mppm)₂(acac)] 2mDBTPDBq- BPhen LiF Al emitting110 nm (=4:2) 20 nm (=0.8:0.2:0.05) II 20 nm 1 nm 200 nm element 40 nm40 nm 10 nm 10

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 40 shows current density-luminance characteristics of thelight-emitting element 9 and the light-emitting element 10. In FIG. 40,the horizontal axis represents current density (mA/cm²), and thevertical axis represents luminance (cd/m²). FIG. 41 showsvoltage-luminance characteristics thereof. In FIG. 41, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 42 shows luminance-current efficiency characteristicsthereof. In FIG. 42, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 43shows luminance-external quantum efficiency characteristics thereof. InFIG. 43, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 12 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 9 and the light-emitting element 10 at aluminance of around 1000 cd/m².

TABLE 12 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 3.2 1.6 (0.43,0.56) 1100 69 68 20 element 9 light-emitting 3.0 1.1 (0.43, 0.56) 860 7579 21 element 10

FIG. 44 shows emission spectra of the light-emitting element 9 and thelight-emitting element 10 which were obtained by applying a current of0.1 mA. In FIG. 44, the horizontal axis represents wavelength (nm), andthe vertical axis represents emission intensity (arbitrary unit). Asshown in Table 12, the CIE chromaticity coordinates of thelight-emitting element 9 at a luminance of 1100 cd/m² were (x, y)=(0.43,0.56), and the CIE chromaticity coordinates of the light-emittingelement 10 at a luminance of 860 cd/m² were (x, y)=(0.43, 0.56). Theseresults show that yellow-green light emission originating from[Ir(mppm)₂(acac)] was obtained from the light-emitting element 9 and thelight-emitting element 10.

As can be seen from Table 12 and FIGS. 40 to 43, the light-emittingelement 9 and the light-emitting element 10 each have high currentefficiency, high power efficiency, and high external quantum efficiency.

In each of the light-emitting element 9 and the light-emitting element10, PCBA1BP or αNBA1BP, 2mDBTPDBq-II, and [Ir(mppm)₂(acac)] described inExample 11 are used for the light-emitting layer. As described inExample 11, the emission spectrum of the mixed material of 2mDBTPDBq-IIand PCBA1BP or αNBA1BP (the emission spectrum of an exciplex) has alarge overlap with the absorption band in the absorption spectrum of[Ir(mppm)₂(acac)] which is considered to greatly contribute to lightemission. The light-emitting element 9 and the light-emitting element 10are considered to have high energy transfer efficiency because they eachtransfer energy by utilizing the overlap, and therefore have highexternal quantum efficiency.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Next, the light-emitting element 9 and the light-emitting element 10were subjected to reliability tests. Results of the reliability testsare shown in FIG. 45. In FIG. 45, the vertical axis representsnormalized luminance (%) with an initial luminance of 100%, and thehorizontal axis represents driving time (h) of the elements.

In the reliability tests, the light-emitting element 9 and thelight-emitting element 10 were driven under the conditions where theinitial luminance was set to 5000 cd/m² and the current density wasconstant.

The luminance of the light-emitting element 9 after 260 hours was 74% ofthe initial luminance. The luminance of the light-emitting element 10after 260 hours was 75% of the initial luminance. From the results, thelight-emitting element 9 and the light-emitting element 10 are found tohave a long lifetime.

The above results show that an element having high reliability can beobtained by application of one embodiment of the present invention.

Example 13

In this example, examples of combinations of a first organic compound, asecond organic compound, and a phosphorescent compound which can be usedfor a light-emitting element of one embodiment of the present inventionwill be described with reference to FIGS. 46A and 46B.

The phosphorescent compound used in this example is(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]). The first organic compound used inthis example is 2mDBTPDBq-II. Two kinds of second organic compounds, NPBand2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF), are used in this example. Chemical formulae ofthe materials used in this example are shown below. Note that thechemical formulae of the materials used in the above examples areomitted here.

<Absorption Spectrum>

FIGS. 46A and 46B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(tBuppm)₂(acac)] that is the phosphorescent compound in adichloromethane solution of [Ir(tBuppm)₂(acac)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 46A and 46B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum19), an emission spectrum of a thin film of DPA2SF that is the secondorganic compound (an emission spectrum 20), an emission spectrum of athin film of a mixed material of 2mDBTPDBq-II and DPA2SF (an emissionspectrum 21), and an emission spectrum of a thin film of a mixedmaterial of 2mDBTPDBq-II and NPB (an emission spectrum 22). In FIG. 46A,the horizontal axis represents wavelength (nm), and the vertical axesrepresent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit). In FIG. 46B, the horizontal axis representsenergy (eV), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 46A,[Ir(tBuppm)₂(acac)] has a broad absorption band at around 490 nm. Thisabsorption band is considered to greatly contribute to light emission.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and DPA2SF (the emission spectrum 21) peaks at a longerwavelength (lower energy) than the emission spectrum of either organiccompound alone. This indicates that an exciplex is formed by mixing2mDBTPDBq-II with DPA2SF.

It is known that the emission spectrum of NPB alone peaks at around 430nm. FIG. 46A indicates that because the emission spectrum of the mixedmaterial of 2mDBTPDBq-II and NPB (the emission spectrum 22) peaks at awavelength longer than 430 nm, an exciplex is formed by mixing2mDBTPDBq-II with NPB.

It is found that the peak of the emission spectrum of the mixed materialhas a large overlap with the absorption band in the absorption spectrumof [Ir(tBuppm)₂(acac)] which is considered to greatly contribute tolight emission. Thus, it is indicated that the light-emitting elementincluding [Ir(tBuppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andDPA2SF and the light-emitting element including [Ir(tBuppm)₂(acac)] andthe mixed material of 2mDBTPDBq-II and NPB have high energy transferefficiency because they each transfer energy by utilizing the overlapbetween the emission spectrum of the mixed material and the absorptionspectrum of the phosphorescent compound. Accordingly, it is indicatedthat a light-emitting element having high external quantum efficiencycan be obtained.

The emission spectrum 22 peaks at a shorter wavelength (higher energy)than the emission spectrum 21. In addition, the peak of the emissionspectrum 22 is closer to the absorption band than the peak of theemission spectrum 21. The above findings show that in FIGS. 46A and 46B,the emission spectrum 22 has the largest overlap with the absorptionband in the absorption spectrum which greatly contributes to lightemission. Specifically, the difference between the peak of theabsorption band in the absorption spectrum and the peak of the emissionspectrum 21 is 0.39 eV, and the difference between the peak of theabsorption band in the absorption spectrum and the peak of the emissionspectrum 22 is 0.19 eV.

The difference between the peaks of the emission spectrum 21 and theemission spectrum 22 is considered to be due to the difference betweenthe HOMO levels of DPA2SF and NPB used as the second organic compounds.Specifically, the HOMO level of DPA2SF is −5.09 eV, whereas the HOMOlevel of NPB is −5.38 eV (each calculated by cyclic voltammetry (CV)measurement). Because NPB has a lower (deeper) HOMO level than DPA2SF,the emission spectrum 22 is considered to peak at a shorter wavelength(higher energy) than the emission spectrum 21.

The above description indicates that the light-emitting elementincluding [Ir(tBuppm)₂(acac)] and the mixed material of 2mDBTPDBq-II andNPB has higher energy transfer efficiency than the light-emittingelement including [Ir(tBuppm)₂(acac)] and the mixed material of2mDBTPDBq-II and DPA2SF because the former transfers energy by utilizinga larger overlap between the emission spectrum of the mixed material andthe absorption spectrum of the phosphorescent compound. Accordingly, itis indicated that a light-emitting element having high external quantumefficiency can be obtained.

In addition, this example reveals that the overlap between the emissionspectrum of the mixed material of the first organic compound and thesecond organic compound and the absorption spectrum of thephosphorescent compound can be increased simply by changing the secondorganic compound without changing the first organic compound. In otherwords, it is indicated that a combination capable of increasing theoverlap can be obtained simply by changing the second organic compoundeven in the case of changing the emission color of the phosphorescentcompound (the position of the absorption band in the absorption spectrumwhich greatly contributes to light emission), and that a light-emittingelement having high external quantum efficiency and long lifetime can beobtained.

Example 14

In this example, light-emitting elements of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

Methods for manufacturing a light-emitting element 11 and alight-emitting element 12 of this example will be described below.

(Light-Emitting Element 11)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, DBT3P-II and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2(=DBT3P-II:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, DPA2SF, and [Ir(tBuppm)₂(acac)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toDPA2SF and [Ir(tBuppm)₂(acac)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:DPA2SF:[Ir(tBuppm)₂(acac)]). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 10 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 20 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 11 of this example was fabricated.

(Light-Emitting Element 12)

A light-emitting layer 1113 of the light-emitting element 12 was formedby co-evaporating 2mDBTPDBq-II, NPB, and [Ir(tBuppm)₂(acac)]. Here, theweight ratio of 2mDBTPDBq-II to NPB and [Ir(tBuppm)₂(acac)] was adjustedto 0.8:0.2:0.05 (=2mDBTPDBq-II:NPB:[Ir(tBuppm)₂(acac)]). The thicknessof the light-emitting layer 1113 was set to 40 nm. Components other thanthe light-emitting layer 1113 were manufactured in a manner similar tothat of the light-emitting element 11.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 13 shows element structures of the light-emitting element 11 andthe light-emitting element 12 obtained as described above.

TABLE 13 first second hole- electron- electron- electron- second firsthole-injection transport transport transport injection elec- elecrtrodelayer layer light-emitting layer layer layer layer trode light- ITSODBT3P-II:MoOx BPAFLP 2mDBTPDBq-II:DPA2SF:[Ir(tBuppm)₂(acac)] 2mDBTPDBq-BPhen LiF Al emitting 110 nm (=4:2) 20 nm (=0.8:0.2:0.05) II 20 nm 1 nm200 nm element 40 nm 40 nm 10 nm 11 light- ITSO DBT3P-II:MoOx BPAFLP2mDBTPDBq-II:NPB:[Ir(tBuppm)₂(acac)] 2mDBTPDBq- BPhen LiF Al emitting110 nm (=4:2) 20 nm (=0.8:0.2:0.05) II 20 nm 1 nm 200 nm element 40 nm40 nm 10 nm 12

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 47 shows current density-luminance characteristics of thelight-emitting element 11 and the light-emitting element 12. In FIG. 47,the horizontal axis represents current density (mA/cm²), and thevertical axis represents luminance (cd/m²). FIG. 48 showsvoltage-luminance characteristics thereof. In FIG. 48, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 49 shows luminance-current efficiency characteristicsthereof. In FIG. 49, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 50shows luminance-external quantum efficiency characteristics thereof. InFIG. 50, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 14 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 11 and the light-emitting element 12 at aluminance of around 1000 cd/m².

TABLE 14 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 2.9 1.7 (0.43,0.56) 890 52 56 15 element 11 light-emitting 2.8 1.0 (0.42, 0.57) 820 8495 23 element 12

FIG. 51 shows emission spectra of the light-emitting element 11 and thelight-emitting element 12 which were obtained by applying a current of0.1 mA. In FIG. 51, the horizontal axis represents wavelength (nm), andthe vertical axis represents emission intensity (arbitrary unit). Asshown in Table 14, the CIE chromaticity coordinates of thelight-emitting element 11 at a luminance of 890 cd/m² were (x, y)=(0.43,0.56), and the CIE chromaticity coordinates of the light-emittingelement 12 at a luminance of 820 cd/m² were (x, y)=(0.42, 0.57). Theseresults show that yellow-green light emission originating from[Ir(tBuppm)₂(acac)] was obtained from the light-emitting element 11 andthe light-emitting element 12.

As can be seen from Table 14 and FIGS. 47 to 50, the light-emittingelement 11 and the light-emitting element 12 each have high currentefficiency, high power efficiency, and high external quantum efficiency.

In each of the light-emitting element 11 and the light-emitting element12, DPA2SF or NPB, 2mDBTPDBq-II, and [Ir(tBuppm)₂(acac)] described inExample 13 are used for the light-emitting layer. As described inExample 13, the emission spectrum of the mixed material of 2mDBTPDBq-IIand DPA2SF or NPB (the emission spectrum of an exciplex) has a largeoverlap with the absorption band in the absorption spectrum of[Ir(tBuppm)₂(acac)] which is considered to greatly contribute to lightemission. The light-emitting element 11 and the light-emitting element12 are considered to have high energy transfer efficiency because theyeach transfer energy by utilizing the overlap, and therefore have highexternal quantum efficiency. In particular, the emission spectrum of themixed material of 2mDBTPDBq-II and NPB has a larger overlap with theabsorption band than the emission spectrum of the mixed material of2mDBTPDBq-II and DPA2SF. Therefore, the light-emitting element 12 isconsidered to have higher energy transfer efficiency than thelight-emitting element 11 because the former transfers energy byutilizing the larger overlap, and therefore have high quantumefficiency. With reference to the results in Example 13 as well, it isfound that the difference between the energy of the peak of the emissionspectrum of an exciplex and the energy of the peak of the absorptionband on the lowest energy side in the absorption spectrum is preferably0.3 eV or less.

In this example, a light-emitting element having further high externalquantum efficiency is obtained simply by changing the second organiccompound (using NPB instead of DPA2SF) without changing the firstorganic compound (corresponding to 2mDBTPDBq-II).

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Example 15

In this example, examples of combinations of a first organic compound, asecond organic compound, and a phosphorescent compound which can be usedfor a light-emitting element of one embodiment of the present inventionwill be described with reference to FIGS. 52A and 52B.

The phosphorescent compound used in this example is[Ir(mppr-Me)₂(dpm)]). Two kinds of first organic compounds, 2mDBTPDBq-IIand 2-[4-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzoimidazole(abbreviation: DBTBIm-II), are used in this example. The second organiccompound used in this example is4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA). Chemical formulae of the materials used in this example areshown below. Note that the chemical formulae of the materials used inthe above example are omitted here.

<Absorption Spectrum>

FIGS. 52A and 52B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(mppr-Me)₂(dpm)] that is the phosphorescent compound in adichloromethane solution of [Ir(mppr-Me)₂(dpm)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 52A and 52B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum23), an emission spectrum of a thin film of DBTBIm-II that is the firstorganic compound (an emission spectrum 24), an emission spectrum of athin film of 1′-TNATA that is the second organic compound (an emissionspectrum 25), an emission spectrum of a thin film of a mixed material of2mDBTPDBq-II and 1′-TNATA (an emission spectrum 26), and an emissionspectrum of a thin film of a mixed material of DBTBIm-II and 1′-TNATA(an emission spectrum 27). In FIG. 52A, the horizontal axis representswavelength (nm), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). InFIG. 52B, the horizontal axis represents energy (eV), and the verticalaxes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 52A,[Ir(mppr-Me)₂(dpm)] has a broad absorption band at around 520 nm. Thisabsorption band is considered to greatly contribute to light emission.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and 1′-TNATA (the emission spectrum 26) and the emissionspectrum of the mixed material of DBTBIm-II and 1′-TNATA (the emissionspectrum 27) each peak at a longer wavelength (lower energy) than theemission spectrum of either organic compound alone. This indicates thatan exciplex is formed by mixing 2mDBTPDBq-II with 1′-TNATA. It alsoindicates that an exciplex is formed by mixing DBTBIm-II with 1′-TNATA.

It is found that the peak of the emission spectrum of each mixedmaterial has a large overlap with the absorption band in the absorptionspectrum of [Ir(mppr-Me)₂(dpm)] which is considered to greatlycontribute to light emission. Thus, it is indicated that thelight-emitting element including [Ir(mppr-Me)₂(dpm)] and the mixedmaterial of 2mDBTPDBq-II and 1′-TNATA and the light-emitting elementincluding [Ir(mppr-Me)₂(dpm)] and the mixed material of DBTBIm-II and1′-TNATA have high energy transfer efficiency because they each transferenergy by utilizing the overlap between the emission spectrum of themixed material and the absorption spectrum of the phosphorescentcompound. Accordingly, it is indicated that a light-emitting elementhaving high external quantum efficiency can be obtained.

The emission spectrum 27 peaks at a shorter wavelength (higher energy)than the emission spectrum 26. In addition, the peak of the emissionspectrum 27 is closer to the absorption band than the peak of theemission spectrum 26. The above findings show that in FIGS. 52A and 52B,the emission spectrum 27 has the largest overlap with the absorptionband in the absorption spectrum which greatly contributes to lightemission. Specifically, the difference between the peak of theabsorption band in the absorption spectrum (a shoulder peak at around520 nm) and the peak of the emission spectrum 26 is 0.35 eV, and thedifference between the peak of the absorption band in the absorptionspectrum (a shoulder peak at around 520 nm) and the peak of the emissionspectrum 27 is 0.01 eV.

The difference between the peaks of the emission spectrum 26 and theemission spectrum 27 is considered to be due to the difference betweenthe LUMO levels of 2mDBTPDBq-II used as the first organic compound andDBTBIm-II. Specifically, the LUMO level of 2mDBTPDBq-II is −2.95 eV,whereas the LUMO level of DBTBIm-II is −2.52 eV (each calculated bycyclic voltammetry (CV) measurement). Because DBTBIm-II has a higher(shallower) LUMO level than 2mDBTPDBq-II, it is considered that theemission spectrum of the mixed material of DBTBIm-II with 1′-TNATAhaving a high HOMO level does not peak at too long a wavelength (i.e.,the emission spectrum 27 peaks at a shorter wavelength than the emissionspectrum 26.

The above description indicates that the light-emitting elementincluding [Ir(mppr-Me)₂(dpm)] and the mixed material of DBTBIm-II and1′-TNATA has higher energy transfer efficiency than the light-emittingelement including [Ir(mppr-Me)₂(dpm)] and the mixed material of2mDBTPDBq-II and 1′-TNATA because the former transfers energy byutilizing a larger overlap between the emission spectrum of the mixedmaterial and the absorption spectrum of the phosphorescent compound.Accordingly, it is indicated that a light-emitting element having highexternal quantum efficiency can be obtained.

In addition, this example reveals that the overlap between the emissionspectrum of the mixed material of the first organic compound and thesecond organic compound and the absorption spectrum of thephosphorescent compound can be increased simply by changing the firstorganic compound without changing the second organic compound. In otherwords, it is indicated that a combination capable of increasing theoverlap can be obtained simply by changing the first organic compoundeven in the case of changing the emission color of the phosphorescentcompound (the position of the absorption band in the absorption spectrumwhich greatly contributes to light emission), and that a light-emittingelement having high external quantum efficiency and long lifetime can beobtained.

Example 16

In this example, light-emitting elements of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

Methods for manufacturing a light-emitting element 13 and alight-emitting element 14 of this example will be described below.

(Light-Emitting Element 13)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, 1′-TNATA, and [Ir(mppr-Me)₂(dpm)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II to1′-TNATA and [Ir(mppr-Me)₂(dpm)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:1′-TNATA:[Ir(mppr-Me)₂(dpm)]). The thickness of thelight-emitting layer 1113 was set to 20 nm.

Further, over the light-emitting layer 1113, a film of 2mDBTPDBq-II wasformed to a thickness of 30 nm to form a first electron-transport layer1114 a.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 20 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 13 of this example was fabricated.

(Light-Emitting Element 14)

A light-emitting layer 1113 of the light-emitting element 14 was formedby co-evaporating DBTBIm-II, 1′-TNATA, and [Ir(mppr-Me)₂(dpm)]. Here,the weight ratio of DBTBIm-II to 1′-TNATA and [Ir(mppr-Me)₂(dpm)] wasadjusted to 0.8:0.2:0.05 (=DBTBIm-II:1′-TNATA:[Ir(mppr-Me)₂(dpm)]). Thethickness of the light-emitting layer 1113 was set to 20 nm.

A first electron-transport layer 1114 a of the light-emitting element 14was formed by forming a film of DBTBIm-II to a thickness of 30 nm.Components other than the light-emitting layer 1113 and the firstelectron-transport layer 1114 a were manufactured in a manner similar tothat of the light-emitting element 13.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 15 shows element structures of the light-emitting element 13 andthe light-emitting element 14 obtained as described above.

TABLE 15 first second hole- electron- electron- electron- firsthole-injection transport transport transport injection second elecrtrodelayer layer light-emitting layer layer layer layer electrode light- ITSOBPAFLP:MoOx BPAFLP 2mDBTPDBq-II:1′-TNATA:[Ir(mppr- 2mDBTPDBq- BPhen LiFAl emitting 110 nm (=4:2) 20 nm Me)₂(dpm)] II 20 nm 1 nm 200 nm element40 nm (=0.8:0.2:0.05) 30 nm 13 20 nm light- ITSO BPAFLP:MoOx BPAFLPDBTBIm-II:1′-TNATA:[Ir(mppr-Me)₂(dpm)] DBTBIm-II BPhen LiF Al emitting110 nm (=4:2) 20 nm (=0.8:0.2:0.05) 30 nm 20 nm 1 nm 200 nm element 40nm 20 nm 14

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 53 shows current density-luminance characteristics of thelight-emitting element 13 and the light-emitting element 14. In FIG. 53,the horizontal axis represents current density (mA/cm²), and thevertical axis represents luminance (cd/m²). FIG. 54 showsvoltage-luminance characteristics thereof. In FIG. 54, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 55 shows luminance-current efficiency characteristicsthereof. In FIG. 55, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 56shows luminance-external quantum efficiency characteristics thereof. InFIG. 56, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 16 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 13 and the light-emitting element 14 at aluminance of 860 cd/m².

TABLE 16 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 3.1 2.9 (0.53,0.46) 860 29 30 11 element 13 light-emitting 3.8 1.5 (0.53, 0.46) 860 5848 21 element 14

FIG. 57 shows emission spectra of the light-emitting element 13 and thelight-emitting element 14 which were obtained by applying a current of0.1 mA. In FIG. 57, the horizontal axis represents wavelength (nm), andthe vertical axis represents emission intensity (arbitrary unit). Asshown in Table 16, the CIE chromaticity coordinates of each of thelight-emitting element 13 and the light-emitting element 14 at aluminance of 860 cd/m² were (x, y)=(0.53, 0.46). These results show thatorange light emission originating from [Ir(mppr-Me)₂(dpm)] was obtainedfrom the light-emitting element 13 and the light-emitting element 14.

As can be seen from Table 16 and FIGS. 53 to 56, the light-emittingelement 13 and the light-emitting element 14 each have high currentefficiency, high power efficiency, and high external quantum efficiency.

In each of the light-emitting element 13 and the light-emitting element14, 2mDBTPDBq-II or DBTBIm-II, 1′-TNATA, and [Ir(mppr-Me)₂(dpm)]described in Example 15 are used for the light-emitting layer. Asdescribed in Example 15, the emission spectrum of the mixed material of2mDBTPDBq-II or DBTBIm-II and 1′-TNATA (the emission spectrum of anexciplex) has a large overlap with the absorption band in the absorptionspectrum of [Ir(mppr-Me)₂(dpm)] which is considered to greatlycontribute to light emission. The light-emitting element 13 and thelight-emitting element 14 are considered to have high energy transferefficiency because they each transfer energy by utilizing the overlap,and therefore have high external quantum efficiency. In particular, theemission spectrum of the mixed material of DBTBIm-II and 1′-TNATA has alarger overlap with the absorption band than the emission spectrum ofthe mixed material of 2mDBTPDBq-II and 1′-TNATA. Therefore, thelight-emitting element 14 is considered to have higher energy transferefficiency than the light-emitting element 13 because the formertransfers energy by utilizing the larger overlap, and therefore havehigh quantum efficiency. With reference to the results in Example 15 aswell, it is found that the difference between the energy of the peak ofthe emission spectrum of an exciplex and the energy of the peak of theabsorption band on the lowest energy side in the absorption spectrum ispreferably 0.3 eV or less.

In this example, a light-emitting element having further high externalquantum efficiency is obtained simply by changing the first organiccompound (using DBTBIm-II instead of 2mDBTPDBq-II) without changing thesecond organic compound (corresponding to 1′-TNATA).

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Example 17

In this example, examples of combinations of a first organic compound, asecond organic compound, and a phosphorescent compound which can be usedfor a light-emitting element of one embodiment of the present inventionwill be described with reference to FIGS. 58A and 58B.

The phosphorescent compound used in this example is [Ir(mppr-Me)₂(dpm)].The first organic compound used in this example is 2mDBTPDBq-II. Twokinds of second organic compounds, PCBNBB and9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP),are used in this example. A chemical formula of a material used in thisexample is shown below. Note that structural formulae of the materialsused in the above examples are omitted here.

<Absorption Spectrum>

FIGS. 58A and 58B each show an ultraviolet-visible absorption spectrum(hereinafter, simply referred to as absorption spectrum) of[Ir(mppr-Me)₂(dpm)] that is the phosphorescent compound in adichloromethane solution of [Ir(mppr-Me)₂(dpm)]. The absorption spectrumwas measured with the use of an ultraviolet-visible lightspectrophotometer (V-550, manufactured by JASCO Corporation) in thestate where the dichloromethane solution (0.093 mmol/L) was put in aquartz cell at room temperature.

<Emission Spectrum>

FIGS. 58A and 58B each also show an emission spectrum of a thin film of2mDBTPDBq-II that is the first organic compound (an emission spectrum28), an emission spectrum of a thin film of PCBNBB that is the secondorganic compound (an emission spectrum 29), an emission spectrum of athin film of PCCP that is the second organic compound (an emissionspectrum 30), an emission spectrum of a thin film of a mixed material of2mDBTPDBq-II and PCBNBB (an emission spectrum 31), and an emissionspectrum of a thin film of a mixed material of 2mDBTPDBq-II and PCCP (anemission spectrum 32). In FIG. 58A, the horizontal axis representswavelength (nm), and the vertical axes represent molar absorptioncoefficient ε (M⁻¹·cm⁻¹) and emission intensity (arbitrary unit). InFIG. 58B, the horizontal axis represents energy (eV), and the verticalaxes represent molar absorption coefficient ε (M⁻¹·cm⁻¹) and emissionintensity (arbitrary unit).

As can be seen from the absorption spectrum in FIG. 58A,[Ir(mppr-Me)₂(dpm)] has a broad absorption band at around 500 nm. Thisabsorption band is considered to greatly contribute to light emission.

It is found that the emission spectrum of the mixed material of2mDBTPDBq-II and PCBNBB (an emission spectrum 31) and the emissionspectrum of the mixed material of 2mDBTPDBq-II and PCCP (an emissionspectrum 32) each peak at a longer wavelength (lower energy) than theemission spectrum of either organic compound alone. This indicates thatan exciplex is formed by mixing 2mDBTPDBq-II with PCBNBB. It alsoindicates that an exciplex is formed by mixing 2mDBTPDBq-II with PCCP.

It is found that the peak of the emission spectrum of each mixedmaterial has a large overlap with the absorption band in the absorptionspectrum of [Ir(mppr-Me)₂(dpm)] which is considered to greatlycontribute to light emission. Thus, it is indicated that thelight-emitting element including [Ir(mppr-Me)₂(dpm)] and the mixedmaterial of 2mDBTPDBq-II and PCBNBB and the light-emitting elementincluding [Ir(mppr-Me)₂(dpm)] and the mixed material of 2mDBTPDBq-II andPCCP have high energy transfer efficiency because they each transferenergy by utilizing the overlap between the emission spectrum of themixed material and the absorption spectrum of the phosphorescentcompound. Accordingly, it is indicated that a light-emitting elementhaving high external quantum efficiency can be obtained.

In addition, this example indicates that either an aromatic amine basedcompound or a carbazole compound can be used as one of the first andsecond organic compounds which form an exciplex.

Example 18

In this example, light-emitting elements of one embodiment of thepresent invention will be described with reference to FIG. 15. Thematerials used in this example are used in the above examples, andtherefore the chemical formulae thereof are omitted here.

Methods for manufacturing a light-emitting element 15 and alight-emitting element 16 of this example will be described below.

(Light-Emitting Element 15)

First, a film of ITSO was formed over a glass substrate 1100 by asputtering method, so that a first electrode 1101 functioning as ananode was formed. Note that the thickness was set to 110 nm and theelectrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 provided with the first electrode 1101 wasfixed to a substrate holder in the vacuum evaporation apparatus so thata surface on which the first electrode 1101 was provided faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, BPAFLP and molybdenum(VI) oxide were co-evaporated toform a hole-injection layer 1111 over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 40 nm, and theweight ratio of BPAFLP to molybdenum oxide was adjusted to 4:2(=BPAFLP:molybdenum oxide).

Next, over the hole-injection layer 1111, a film of BPAFLP was formed toa thickness of 20 nm to form a hole-transport layer 1112.

Furthermore, 2mDBTPDBq-II, PCBNBB, and [Ir(mppr-Me)₂(dpm)] wereco-evaporated to form a light-emitting layer 1113 over thehole-transport layer 1112. Here, the weight ratio of 2mDBTPDBq-II toPCBNBB and [Ir(mppr-Me)₂(dpm)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-II:PCBNBB:[Ir(mppr-Me)₂(dpm)]). The thickness of thelight-emitting layer 1113 was set to 20 nm.

Further, over the light-emitting layer 1113, 2mDBTPDBq-II, PCBNBB, and[Ir(mppr-Me)₂(dpm)] were co-evaporated to form a firstelectron-transport layer 1114 a over the light-emitting layer 1113.Here, the weight ratio of 2mDBTPDBq-III to PCBNBB and[Ir(mppr-Me)₂(dpm)] was adjusted to 0.8:0.2:0.05(=2mDBTPDBq-III:PCBNBB:[Ir(mppr-Me)₂(dpm)]). The thickness of the firstelectron-transport layer 1114 a was set to 40 nm.

Next, over the first electron-transport layer 1114 a, a film of BPhenwas formed to a thickness of 10 nm to form a second electron-transportlayer 1114 b.

Further, over the second electron-transport layer 1114 b, a film of LiFwas formed by evaporation to a thickness of 1 nm to form anelectron-injection layer 1115.

Lastly, an aluminum film was formed by evaporation to a thickness of 200nm as a second electrode 1103 functioning as a cathode. Thus, thelight-emitting element 15 of this example was fabricated.

(Light-Emitting Element 16)

A light-emitting layer 1113 of the light-emitting element 16 was formedby co-evaporating 2mDBTPDBq-II, PCCP, and [Ir(mppr-Me)₂(dpm)]. Here, theweight ratio of 2mDBTPDBq-II to PCCP and [Ir(mppr-Me)₂(dpm)] wasadjusted to 0.8:0.2:0.05 (=2mDBTPDBq-II:PCCP:[Ir(mppr-Me)₂(dpm)]). Thethickness of the light-emitting layer 1113 was set to 20 nm. Componentsother than the light-emitting layer 1113 were manufactured in a mannersimilar to that of the light-emitting element 15.

Note that, in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 17 shows element structures of the light-emitting element 15 andthe light-emitting element 16 obtained as described above.

TABLE 17 hole- first hole-injection transport elecrtrode layer layerlight-emitting layer light- ITSO BPAFLP:MoOx BPAFLP2mDBTPDBq-II:PCBNBB:[Ir(mppr- emitting 110 nm (=4:2) 20 nm Me)₂(dpm)]element 15 40 nm (=0.8:0.2:0.05) 20 nm light- ITSO BPAFLP:MoOx BPAFLP2mDBTPDBq-II:PCCP:[Ir(mppr- emitting 110 nm (=4:2) 20 nm Me)₂(dpm)]element 16 40 nm (=0.8:0.2:0.05) 20 nm second electron- electron- firstelectron-transport transport injection second layer layer layerelectrode light- 2mDBTPDBq-II:PCBNBB:[Ir(mppr- BPhen LiF Al emittingMe)₂(dpm)] 10 nm 1 nm 200 nm element 15 (=0.8:0.2:0.05) 40 nm light-2mDBTPDBq-II:PCBNBB:[Ir(mppr- BPhen LiF Al emitting Me)₂(dpm)] 10 nm 1nm 200 nm element 16 (=0.8:0.2:0.05) 40 nm

In a glove box containing a nitrogen atmosphere, these light-emittingelements were sealed so as not to be exposed to air. Then, operationcharacteristics of these light-emitting elements were measured. Notethat the measurements were carried out at room temperature (in theatmosphere kept at 25° C.).

FIG. 59 shows current density-luminance characteristics of thelight-emitting element 15 and the light-emitting element 16. In FIG. 59,the horizontal axis represents current density (mA/cm²), and thevertical axis represents luminance (cd/m²). FIG. 60 showsvoltage-luminance characteristics thereof. In FIG. 60, the horizontalaxis represents voltage (V), and the vertical axis represents luminance(cd/m²). FIG. 61 shows luminance-current efficiency characteristicsthereof. In FIG. 61, the horizontal axis represents luminance (cd/m²),and the vertical axis represents current efficiency (cd/A). FIG. 62shows luminance-external quantum efficiency characteristics thereof. InFIG. 62, the horizontal axis represents luminance (cd/m²), and thevertical axis represents external quantum efficiency (%).

Further, Table 18 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 15 and the light-emitting element 16 at aluminance of 1200 cd/m².

TABLE 18 external current chromaticity current power quantum voltagedensity coordinates luminance efficiency efficiency efficiency (V)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) light-emitting 3.0 1.7 (0.54,0.45) 1200 67 70 24 element 15 light-emitting 3.0 1.7 (0.54, 0.46) 120069 72 24 element 16

FIG. 63 shows emission spectra of the light-emitting element 15 and thelight-emitting element 16 which were obtained by applying a current of0.1 mA. In FIG. 63, the horizontal axis represents wavelength (nm), andthe vertical axis represents emission intensity (arbitrary unit). Asshown in Table 18, the CIE chromaticity coordinates of thelight-emitting element 15 at a luminance of 1200 cd/m² were (x,y)=(0.54, 0.45), and the CIE chromaticity coordinates of thelight-emitting element 16 at a luminance of 1200 cd/m² were (x,y)=(0.54, 0.46). These results show that orange light emissionoriginating from [Ir(mppr-Me)₂(dpm)] was obtained from thelight-emitting element 15 and the light-emitting element 16.

As can be seen from Table 18 and FIGS. 59 to 62, the light-emittingelement 15 and the light-emitting element 16 each have high currentefficiency, high power efficiency, and high external quantum efficiency.

In each of the light-emitting element 15 and the light-emitting element16, 2mDBTPDBq-II, PCBNBB or PCCP, and [Ir(mppr-Me)₂(dpm)] described inExample 17 are used for the light-emitting layer. As described inExample 17, the emission spectrum of the mixed material of 2mDBTPDBq-IIand PCBNBB or PCCP (the emission spectrum of an exciplex) has a largeoverlap with the absorption band in the absorption spectrum of[Ir(mppr-Me)₂(dpm)] which is considered to greatly contribute to lightemission. The light-emitting element 15 and the light-emitting element16 are considered to have high energy transfer efficiency because theyeach transfer energy by utilizing the overlap, and therefore have highexternal quantum efficiency.

In addition, this example shows that an exciplex can be formed by usingeither an aromatic amine based compound (PCBNBB) or a carbazole compound(PCCP) and a light-emitting layer having high external quantumefficiency can be obtained.

The above results show that an element having high external quantumefficiency can be obtained by application of one embodiment of thepresent invention.

Reference Example 1

A synthetic example of an organometallic complex(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (anothername: bis[2-(6-phenyl-4-pyrimidinyl-κC3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)) (abbreviation:[Ir(dppm)₂(acac)]), which is used in the above examples, is described.The structure of [Ir(dppm)₂(acac)] is shown below.

Step 1: Synthesis of 4,6-Diphenylpyrimidine (Abbreviation: Hdppm)

First, 5.02 g of 4,6-dichloropyrimidine, 8.29 g of phenylboronic acid,7.19 g of sodium carbonate, 0.29 g ofbis(triphenylphosphine)palladium(II)dichloride (abbreviation:Pd(PPh₃)₂Cl₂), 20 mL of water, and 20 mL of acetonitrile were put into arecovery flask equipped with a reflux pipe, and the air in the flask wasreplaced with argon. This reaction container was heated by irradiationwith microwaves (2.45 GHz, 100 W) for 60 minutes. Here, there werefurther put 2.08 g of phenylboronic acid, 1.79 g of sodium carbonate,0.070 g of Pd(PPh₃)₂Cl₂, 5 mL of water, and 5 mL of acetonitrile intothe flask, and the mixture was heated again by irradiation withmicrowaves (2.45 GHz, 100 W) for 60 minutes. After that, water was addedto this solution and an organic layer was extracted withdichloromethane. The obtained solution of the extract was washed withwater and dried with magnesium sulfate. The solution after drying wasfiltered. The solvent of this solution was distilled off, and then theobtained residue was purified by silica gel column chromatography usingdichloromethane as a developing solvent, so that a pyrimidine derivativeHdppm (yellow white powder, yield of 38%) was obtained. Note that forthe irradiation with microwaves, a microwave synthesis system (Discover,manufactured by CEM Corporation) was used. A synthesis scheme (a-1) ofStep 1 is shown below.

Step 2: Synthesis ofDi-t-chloro-bis[bis(4,6-diphenylpyrimidinato)iridium(III)](Abbreviation:[Ir(dppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.10 g of Hdppm obtainedin Step 1, and 0.69 g of iridium chloride hydrate (IrCl₃.H₂O) were putinto a recovery flask equipped with a reflux pipe, and the air in therecovery flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas filtered and washed with ethanol to give a dinuclear complex[Ir(dppm)₂Cl]₂ (reddish brown powder, yield of 88%). A synthesis scheme(a-2) of Step 2 is shown below.

Step 3: Synthesis of(Acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(Abbreviation: [Ir(dppm)₂(acac)])

Furthermore, 40 mL of 2-ethoxyethanol, 1.44 g of [Ir(dppm)₂Cl]₂ obtainedin Step 2, 0.30 g of acetylacetone, and 1.07 g of sodium carbonate wereput into a recovery flask equipped with a reflux pipe, and the air inthe recovery flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 120 W) was performed for 60 minutes to cause areaction. The solvent was distilled off, the obtained residue wasdissolved in dichloromethane, and filtration was performed to removeinsoluble matter. The obtained filtrate was washed with water and thenwith saturated saline, and was dried with magnesium sulfate. Thesolution after drying was filtered. The solvent of this solution wasdistilled off, and then the obtained residue was purified by silica gelcolumn chromatography using dichloromethane and ethyl acetate as adeveloping solvent in a volume ratio of 50:1. After that,recrystallization was carried out with a mixed solvent ofdichloromethane and hexane, so that the objective orange powder (yieldof 32%) was obtained. A synthesis scheme (a-3) of Step 3 is shown below.

An analysis result by nuclear magnetic resonance spectrometry (¹H NMR)of the orange powder obtained in Step 3 is described below. Theseresults revealed that the organometallic complex [Ir(dppm)₂(acac)] wasobtained.

¹H NMR. δ (CDCl₃): 1.83 (s, 6H), 5.29 (s, 1H), 6.48 (d, 2H), 6.80 (t,2H), 6.90 (t, 2H), 7.55-7.63 (m, 6H), 7.77 (d, 2H), 8.17 (s, 2H), 8.24(d, 4H), 9.17 (s, 2H).

Reference Example 2

A synthetic example of an organometallic complex(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (anothername:bis[2-(6-methyl-4-pyrimidinyl-κC)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III))(abbreviation: [Ir(mppm)₂(acac)]), which is used in the above examples,is specifically described. The structure of [Ir(mppm)₂(acac)] is shownbelow.

Step 1: Synthesis of 4-Methyl-6-phenylpyrimidine (Abbreviation: Hmppm)

First, 4.90 g of 4-chloro-6-methylpyrimidine, 4.80 g of phenylboronicacid, 4.03 g of sodium carbonate, 0.16 g ofbis(triphenylphosphine)palladium(II)dichloride (abbreviation:Pd(PPh₃)₂Cl₂), 20 mL of water, and 10 mL of acetonitrile were put into arecovery flask equipped with a reflux pipe, and the air in the flask wasreplaced with argon. This reaction container was heated by irradiationwith microwaves (2.45 GHz, 100 W) for 60 minutes. Here, there werefurther put 2.28 g of phenylboronic acid, 2.02 g of sodium carbonate,0.082 g of Pd(PPh₃)₂Cl₂, 5 mL of water, and 10 mL of acetonitrile in theflask, and the mixture was heated again by irradiation with microwaves(2.45 GHz, 100 W) for 60 minutes. After that, water was added to thissolution and extraction with dichloromethane was carried out. Theobtained solution of the extract was washed with a saturated aqueoussolution of sodium carbonate, water, and then with saturated saline, anddried with magnesium sulfate. The solution after drying was filtered.The solvent of this solution was distilled off, and then the obtainedresidue was purified by silica gel column chromatography usingdichloromethane and ethyl acetate as a developing solvent in a volumeratio of 9:1, so that the objective pyrimidine derivative Hmppm (orangeoily substance, yield of 46%) was obtained. Note that the irradiationwith microwaves was performed using a microwave synthesis system(Discover, manufactured by CEM Corporation). A synthesis scheme (b-1) ofStep 1 is shown below.

Step 2: Synthesis ofDi-t-chloro-bis[bis(6-methyl-4-phenylpyrimidinato)iridium(III)](Abbreviation:[Ir(mppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.51 g of Hmppm obtainedin Step 1, and 1.26 g of iridium chloride hydrate (IrCl₃.H₂O) were putinto a recovery flask equipped with a reflux pipe, and the air in therecovery flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas washed with ethanol and filtered, so that a dinuclear complex[Ir(mppm)₂Cl]₂ (dark green powder, yield of 77%) was obtained. Asynthesis scheme (b-2) of Step 2 is shown below.

Step 3: Synthesis of(Acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(Abbreviation: [Ir(mppm)₂(acac)])

Furthermore, 40 mL of 2-ethoxyethanol, 1.84 g of the dinuclear complex[Ir(mppm)₂Cl]₂ obtained in Step 2, 0.48 g of acetylacetone, and 1.73 gof sodium carbonate were put into a recovery flask equipped with areflux pipe, and the air in the recovery flask was replaced with argon.After that, irradiation with microwaves (2.45 GHz, 120 W) was performedfor 60 minutes to cause a reaction. The solvent was distilled off, theobtained residue was dissolved in dichloromethane, and filtration wasperformed to remove insoluble matter. The obtained filtrate was washedwith water and then with saturated saline, and was dried with magnesiumsulfate. The solution after drying was filtered. The solvent of thissolution was distilled off, and then the obtained residue was purifiedby silica gel column chromatography using dichloromethane and ethylacetate as a developing solvent in a volume ratio of 4:1. After that,recrystallization was carried out with a mixed solvent ofdichloromethane and hexane, so that the objective yellow powder (yieldof 22%) was obtained. A synthesis scheme (b-3) of Step 3 is shown below.

An analysis result by nuclear magnetic resonance spectrometry (¹H NMR)of the yellow powder obtained in Step 3 is described below. Theseresults revealed that the organometallic complex [Ir(mppm)₂(acac)] wasobtained.

¹H NMR. δ (CDCl₃): 1.78 (s, 6H), 2.81 (s, 6H), 5.24 (s, 1H), 6.37 (d,2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.61-7.63 (m, 4H), 8.97 (s, 2H).

Reference Example 3

A synthetic example of an organometallic complex(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(another name:bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)) (abbreviation: [Ir(tBuppm)₂(acac)]), which is used in the aboveexamples, is described. The structure of [Ir(tBuppm)₂(acac)] is shownbelow.

Step 1: Synthesis of 4-tert-Butyl-6-phenylpyrimidine (abbreviation:HtBuppm)

First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g offormamide were put into a recovery flask equipped with a reflux pipe,and the air in the flask was replaced with nitrogen. This reactioncontainer was heated, so that the reacted solution was refluxed for 5hours. After that, this solution was poured into an aqueous solution ofsodium hydroxide, and an organic layer was extracted withdichloromethane. The obtained organic layer was washed with water andsaturated saline, and dried with magnesium sulfate. The solution afterdrying was filtered. The solvent of this solution was distilled off, andthen the obtained residue was purified by silica gel columnchromatography using hexane and ethyl acetate as a developing solvent ina volume ratio of 10:1, so that a pyrimidine derivative HtBuppm(colorless oily substance, yield of 14%) was obtained. A syntheticscheme (c-1) of Step 1 is shown below.

Step 2: Synthesis ofDi-μ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)](Abbreviation:[Ir(tBuppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of HtBuppmobtained in Step 1, and 1.04 g of iridium chloride hydrate (IrCl₃.H₂O)were put into a recovery flask equipped with a reflux pipe, and the airin the flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas suction-filtered and washed with ethanol, so that a dinuclearcomplex [Ir(tBuppm)₂Cl]₂ (yellow green powder, yield of 73%) wasobtained. A synthesis scheme (c-2) of Step 2 is shown below.

Step 3: Synthesis of(Acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(Abbreviation: [Ir(tBuppm)₂(acac)])

Further, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear complex[Ir(tBuppm)₂Cl]₂ obtained in Step 2, 0.36 g of acetylacetone, and 1.27 gof sodium carbonate were put into a recovery flask equipped with areflux pipe, and the air in the flask was replaced with argon. Afterthat, irradiation with microwaves (2.45 GHz, 120 W) was performed for 60minutes to cause a reaction. The solvent was distilled off, and theobtained residue was suction-filtered with ethanol and washed with waterand ethanol. This solid was dissolved in dichloromethane, and themixture was filtered through a filter aid in which Celite (produced byWako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina,and Celite were stacked in this order. The solvent was distilled off,and the obtained solid was recrystallized with a mixed solvent ofdichloromethane and hexane, so that the objective substance was obtainedas yellow powder (yield of 68%). A synthesis scheme (c-3) of Step 3 isshown below.

An analysis result by nuclear magnetic resonance spectrometry (¹H NMR)of the yellow powder obtained in Step 3 is described below. Theseresults revealed that the organometallic complex [Ir(tBuppm)₂(acac)] wasobtained.

¹H NMR. δ (CDCl₃): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d,2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s,2H).

Reference Example 4

A method of synthesizing2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), which is used in the above examples, is described.

Synthesis of 2-[3-(Dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(Abbreviation: 2mDBTPDBq-II)

A synthesis scheme (d-1) of2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) is shown below.

First, 5.3 g (20 mmol) of 2-chlorodibenzo[f,h]quinoxaline, 6.1 g (20mmol) of 3-(dibenzothiophen-4-yl)phenylboronic acid, 460 mg (0.4 mmol)of tetrakis(triphenylphosphine)palladium(0), 300 mL of toluene, 20 mL ofethanol, and 20 mL of a 2M aqueous solution of potassium carbonate wereput in a 2 L three-neck flask. The mixture was degassed by being stirredunder reduced pressure, and the air in the three-neck flask was replacedwith nitrogen. This mixture was stirred under a nitrogen stream at 100°C. for 7.5 hours. After cooled to room temperature, the obtained mixturewas filtered to give a white residue. The obtained residue was washedwith water and ethanol in this order, and then dried. The obtained solidwas dissolved in about 600 mL of hot toluene, followed by suctionfiltration through Celite and Florisil, whereby a clear colorlessfiltrate was obtained. The obtained filtrate was concentrated andpurified by silica gel column chromatography using about 700 mL ofsilica gel. The chromatography was carried out using hot toluene as adeveloping solvent. Acetone and ethanol were added to the solid obtainedhere, followed by irradiation with ultrasonic waves. Then, the generatedsuspended solid was collected by filtration and the obtained solid wasdried, so that 7.85 g of white powder was obtained in 80% yield.

The above objective substance was relatively soluble in hot toluene, butwas a material that was likely to be precipitated when cooled. Further,the substance was poorly soluble in other organic solvents such asacetone and ethanol. Hence, the utilization of these different degreesof solubility resulted in a high-yield synthesis by a simple method asabove. Specifically, after the reaction finished, the mixture wasreturned to room temperature and the precipitated solid was collected byfiltration, whereby most impurities were able to be easily removed.Further, by the column chromatography with hot toluene as a developingsolvent, the objective substance, which is likely to be precipitated,was able to be readily purified.

By a train sublimation method, 4.0 g of the obtained white powder waspurified. In the purification, the white powder was heated at 300° C.under a pressure of 5.0 Pa with a flow rate of argon gas of 5 mL/min.After the purification, the objective substance was obtained in a yieldof 88% as 3.5 g of white powder.

A nuclear magnetic resonance spectrometry (¹H NMR) identified thiscompound as the objective2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II).

¹H NMR data of the obtained substance are shown below.

¹H NMR (CDCl₃, 300 MHz): δ (ppm)=7.45-7.52 (m, 2H), 7.59-7.65 (m, 2H),7.71-7.91 (m, 7H), 8.20-8.25 (m, 2H), 8.41 (d, J=7.8 Hz, 1H), 8.65 (d,J=7.5 Hz, 2H), 8.77-8.78 (m, 1H), 9.23 (dd, J=7.2 Hz, 1.5 Hz, 1H), 9.42(dd, J=7.8 Hz, 1.5 Hz, 1H), 9.48 (s, 1H).

EXPLANATION OF REFERENCE

102: EL layer, 103: first electrode, 108: second electrode, 701:hole-injection layer, 702: hole-transport layer, 703: light-emittinglayer, 704: electron-transport layer, 705: electron-injection layer,706: electron-injection buffer layer, 707: electron-relay layer, 708:composite material layer, 800: first EL layer, 801: second EL layer,803: charge-generation layer, 1100: substrate, 1101: first electrode,1103: second electrode, 1111: hole-injection layer, 1112: hole-transportlayer, 1113: light-emitting layer, 1114 a: first electron-transportlayer, 1114 b: second electron-transport layer, and 1115:electron-injection layer.

This application is based on Japanese Patent Application serial no.2011-031462 filed with Japan Patent Office on Feb. 16, 2011, the entirecontents of which are hereby incorporated by reference.

1. A method of emitting light from a light-emitting of elementcomprising a first compound, a second compound, and a phosphorescentcompound between a pair of electrodes, the method comprising: forming anexciplex between the first compound and the second compound; andtransferring an excitation energy from the exciplex to thephosphorescent compound, wherein an emission spectrum of the exciplexoverlaps with a longest wavelength side absorption band in an absorptionspectrum of the phosphorescent compound, and wherein each of a level ofa triplet excitation energy of the first compound and a level of atriplet excitation energy of the second compound is higher than a levelof a triplet excitation energy of the phosphorescent compound.
 2. Themethod according to claim 1, wherein a difference between an energyvalue of a peak of the emission spectrum and an energy value of a peakof the longest wavelength side absorption band in the absorptionspectrum is 0.2 eV or less.
 3. The method according to claim 1, whereina difference between an energy value of a peak of the emission spectrumand an energy value of a peak of the longest wavelength side absorptionband in the absorption spectrum is 0.1 eV or less.
 4. The methodaccording to claim 1, wherein the emission spectrum is a fluorescentspectrum.
 5. The method according to claim 1, wherein the phosphorescentcompound is an organometallic complex.
 6. The method according to claim1, wherein the phosphorescent compound is an iridium complex.
 7. Themethod according to claim 1, wherein a molar absorption coefficient ofthe longest wavelength side absorption band in the absorption spectrumis 2000 M⁻¹·cm⁻¹ or higher.
 8. The method according to claim 1, whereinthe first compound comprises a heterocyclic compound, and wherein thesecond compound comprises a carbazole compound.
 9. The method accordingto claim 1, wherein the light-emitting element comprises alight-emitting layer and a hole-injection layer between the pair ofelectrodes, and wherein the light-emitting layer comprises the firstcompound, the second compound, and the phosphorescent compound.
 10. Themethod according to claim 1, wherein the light-emitting elementcomprises a light-emitting layer and a hole-injection layer between thepair of electrodes, wherein the light-emitting layer comprises the firstcompound, the second compound, and the phosphorescent compound, andwherein the hole-injection layer comprises an organic compound and anelectron acceptor.
 11. A method of emitting light from a light-emittingelement comprising a first compound, a second compound, and aphosphorescent compound between a pair of electrodes, the methodcomprising: applying a voltage between the pair of electrodes to form anexciplex between the first compound and the second compound, wherein anexcitation energy of the exciplex is transferred to the phosphorescentcompound, wherein an emission spectrum of the exciplex overlaps with alongest wavelength side absorption band in an absorption spectrum of thephosphorescent compound, and wherein each of a level of a tripletexcitation energy of the first compound and a level of a tripletexcitation energy of the second compound is higher than a level of atriplet excitation energy of the phosphorescent compound.
 12. The methodaccording to claim 11, wherein a difference between an energy value of apeak of the emission spectrum and an energy value of a peak of thelongest wavelength side absorption band in the absorption spectrum is0.2 eV or less.
 13. The method according to claim 11, wherein adifference between an energy value of a peak of the emission spectrumand an energy value of a peak of the longest wavelength side absorptionband in the absorption spectrum is 0.1 eV or less.
 14. The methodaccording to claim 11, wherein the emission spectrum is a fluorescentspectrum.
 15. The method according to claim 11, wherein thephosphorescent compound is an organometallic complex.
 16. The methodaccording to claim 11, wherein the phosphorescent compound is an iridiumcomplex.
 17. The method according to claim 11, wherein a molarabsorption coefficient of the longest wavelength side absorption band inthe absorption spectrum is 2000 M⁻¹·cm⁻¹ or higher.
 18. The methodaccording to claim 11, wherein the first compound comprises aheterocyclic compound, and wherein the second compound comprises acarbazole compound.
 19. The method according to claim 11, wherein thelight-emitting element comprises a light-emitting layer and ahole-injection layer between the pair of electrodes, and wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound.
 20. The method according to claim 11,wherein the light-emitting element comprises a light-emitting layer anda hole-injection layer between the pair of electrodes, wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound, and wherein the hole-injection layercomprises an organic compound and an electron acceptor.
 21. A method ofusing a light-emitting element comprising a first compound, a secondcompound, and a phosphorescent compound between a pair of electrodes,the method comprising: forming an exciplex between the first compoundand the second compound; and transferring an excitation energy from theexciplex to the phosphorescent compound, wherein an emission spectrum ofthe exciplex overlaps with a longest wavelength side absorption band inan absorption spectrum of the phosphorescent compound, and wherein eachof a level of a triplet excitation energy of the first compound and alevel of a triplet excitation energy of the second compound is higherthan a level of a triplet excitation energy of the phosphorescentcompound.
 22. The method according to claim 21, wherein a differencebetween an energy value of a peak of the emission spectrum and an energyvalue of a peak of the longest wavelength side absorption band in theabsorption spectrum is 0.2 eV or less.
 23. The method according to claim21, wherein a difference between an energy value of a peak of theemission spectrum and an energy value of a peak of the longestwavelength side absorption band in the absorption spectrum is 0.1 eV orless.
 24. The method according to claim 21, wherein the emissionspectrum is a fluorescent spectrum.
 25. The method according to claim21, wherein the phosphorescent compound is an organometallic complex.26. The method according to claim 21, wherein the phosphorescentcompound is an iridium complex.
 27. The method according to claim 21,wherein a molar absorption coefficient of the longest wavelength sideabsorption band in the absorption spectrum is 2000 M⁻¹·cm⁻¹ or higher.28. The method according to claim 21, wherein the first compoundcomprises a heterocyclic compound, and wherein the second compoundcomprises a carbazole compound.
 29. The method according to claim 21,wherein the light-emitting element comprises a light-emitting layer anda hole-injection layer between the pair of electrodes, and wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound.
 30. The method according to claim 21,wherein the light-emitting element comprises a light-emitting layer anda hole-injection layer between the pair of electrodes, wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound, and wherein the hole-injection layercomprises an organic compound and an electron acceptor.
 31. The methodaccording to claim 21, wherein the light-emitting element emits light.32. A method of using a light-emitting element comprising a firstcompound, a second compound, and a phosphorescent compound between apair of electrodes, the method comprising: applying a voltage betweenthe pair of electrodes to form an exciplex between the first compoundand the second compound, wherein an excitation energy of the exciplex istransferred to the phosphorescent compound, wherein an emission spectrumof the exciplex overlaps with a longest wavelength side absorption bandin an absorption spectrum of the phosphorescent compound, and whereineach of a level of a triplet excitation energy of the first compound anda level of a triplet excitation energy of the second compound is higherthan a level of a triplet excitation energy of the phosphorescentcompound.
 33. The method according to claim 32, wherein a differencebetween an energy value of a peak of the emission spectrum and an energyvalue of a peak of the longest wavelength side absorption band in theabsorption spectrum is 0.2 eV or less.
 34. The method according to claim32, wherein a difference between an energy value of a peak of theemission spectrum and an energy value of a peak of the longestwavelength side absorption band in the absorption spectrum is 0.1 eV orless.
 35. The method according to claim 32, wherein the emissionspectrum is a fluorescent spectrum.
 36. The method according to claim32, wherein the phosphorescent compound is an organometallic complex.37. The method according to claim 32, wherein the phosphorescentcompound is an iridium complex.
 38. The method according to claim 32,wherein a molar absorption coefficient of the longest wavelength sideabsorption band in the absorption spectrum is 2000 M⁻¹·cm⁻¹ or higher.39. The method according to claim 32, wherein the first compoundcomprises a heterocyclic compound, and wherein the second compoundcomprises a carbazole compound.
 40. The method according to claim 32,wherein the light-emitting element comprises a light-emitting layer anda hole-injection layer between the pair of electrodes, and wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound.
 41. The method according to claim 32,wherein the light-emitting element comprises a light-emitting layer anda hole-injection layer between the pair of electrodes, wherein thelight-emitting layer comprises the first compound, the second compound,and the phosphorescent compound, and wherein the hole-injection layercomprises an organic compound and an electron acceptor.
 42. The methodaccording to claim 32, wherein the light-emitting element emits light.